RelativePanel error: Element does not exist in the current context

[axtens]

Context: Windows 11, Visual Studio 2022 Community, Uno 4. (The Uno 4 community suggested I take this up with microsoft-ui-xaml directly.)

"No installed components were detected." is all the other text that comes with this crash. The code that precipitates it is

        private void HandleOutputPanelChange(string pos)
        {
            var outputPanelPosition = (OutputPanelPosition)Enum.Parse(typeof(OutputPanelPosition), pos);
            switch (outputPanelPosition)
            {
                case OutputPanelPosition.Left:
                    //outputPanelPosition = OutputPanelPosition.Left;

                    RelativePanel.SetAlignLeftWithPanel(outputPanel, true);
                    RelativePanel.SetAlignRightWithPanel(outputPanel, false);
                    RelativePanel.SetBelow(outputPanel, mnuNew);
                    outputPanel.Width = 200;
                    outputPanel.MinWidth = 175;
                    outputPanel.MaxWidth = 700;
                    outputPanel.ClearValue(HeightProperty);
                    outputPanel.ClearValue(MaxHeightProperty);

                    RelativePanel.SetAbove(tabControl, statusBar);
                    RelativePanel.SetRightOf(tabControl, outputPanel);
                    RelativePanel.SetAlignLeftWithPanel(tabControl, false);
                    RelativePanel.SetAlignRightWithPanel(tabControl, true);

                    outputLeftButton.BorderBrush = new SolidColorBrush(Colors.DodgerBlue);
                    outputBottomButton.BorderBrush = new SolidColorBrush(Colors.LightGray);
                    outputRightButton.BorderBrush = new SolidColorBrush(Colors.LightGray);

                    outputLeftButton.Background = new SolidColorBrush(Colors.DeepSkyBlue);
                    outputBottomButton.Background = new SolidColorBrush(Colors.Transparent);
                    outputRightButton.Background = new SolidColorBrush(Colors.Transparent);

                    Canvas.SetLeft(outputThumb, outputPanel.Width - 1);
                    Canvas.SetTop(outputThumb, 0);

                    outputDockingFlyout.Hide();
                    break;
                case OutputPanelPosition.Bottom:
                    //outputPanelPosition = OutputPanelPosition.Bottom;

                    RelativePanel.SetAlignLeftWithPanel(tabControl, true);
                    RelativePanel.SetAlignRightWithPanel(tabControl, true);
                    RelativePanel.SetRightOf(tabControl, null);
                    RelativePanel.SetAbove(tabControl, outputPanel);

                    RelativePanel.SetAlignLeftWithPanel(outputPanel, true);
                    RelativePanel.SetAlignRightWithPanel(outputPanel, true);
                    RelativePanel.SetBelow(outputPanel, null);
                    outputPanel.Height = 200;
                    outputPanel.MaxHeight = 500;
                    outputPanel.ClearValue(WidthProperty);
                    outputPanel.ClearValue(MaxWidthProperty);

                    outputBottomButton.BorderBrush = new SolidColorBrush(Colors.DodgerBlue);
                    outputLeftButton.BorderBrush = new SolidColorBrush(Colors.LightGray);
                    outputRightButton.BorderBrush = new SolidColorBrush(Colors.LightGray);

                    outputBottomButton.Background = new SolidColorBrush(Colors.DeepSkyBlue);
                    outputLeftButton.Background = new SolidColorBrush(Colors.Transparent);
                    outputRightButton.Background = new SolidColorBrush(Colors.Transparent);

                    Canvas.SetLeft(outputThumb, 0);
                    Canvas.SetTop(outputThumb, -4);

                    outputDockingFlyout.Hide();
                    break;
                case OutputPanelPosition.Right:
                    //outputPanelPosition = OutputPanelPosition.Right;

                    RelativePanel.SetAlignLeftWithPanel(outputPanel, false);
                    RelativePanel.SetAlignRightWithPanel(outputPanel, true);
                    RelativePanel.SetBelow(outputPanel, mnuNew);
                    outputPanel.Width = 200;
                    outputPanel.MinWidth = 175;
                    outputPanel.MaxWidth = 700;
                    outputPanel.ClearValue(HeightProperty);
                    outputPanel.ClearValue(MaxHeightProperty);

                    RelativePanel.SetAbove(tabControl, statusBar);
                    RelativePanel.SetLeftOf(tabControl, outputPanel);
                    RelativePanel.SetAlignLeftWithPanel(tabControl, true);
                    RelativePanel.SetAlignRightWithPanel(tabControl, false);

                    outputRightButton.BorderBrush = new SolidColorBrush(Colors.DodgerBlue);
                    outputBottomButton.BorderBrush = new SolidColorBrush(Colors.LightGray);
                    outputLeftButton.BorderBrush = new SolidColorBrush(Colors.LightGray);

                    outputRightButton.Background = new SolidColorBrush(Colors.DeepSkyBlue);
                    outputBottomButton.Background = new SolidColorBrush(Colors.Transparent);
                    outputLeftButton.Background = new SolidColorBrush(Colors.Transparent);

                    Canvas.SetLeft(outputThumb, -4);
                    Canvas.SetTop(outputThumb, 0);

                    outputDockingFlyout.Hide();
                    break;
            }
        }

the attendant XAML is

        <Border x:Name="outputPanel" RelativePanel.Above="statusBar" RelativePanel.AlignLeftWithPanel="True" RelativePanel.AlignRightWithPanel="True" 
                    Height="200" Background="LightGray" MinHeight="100" MaxHeight="500" Margin="5, 0, 5, 0" SizeChanged="OutputPanel_SizeChanged">
            <Canvas HorizontalAlignment="Stretch" VerticalAlignment="Stretch">
                <Thumb x:Name="outputThumb" Height="5" DragDelta="Thumb_DragDelta" DragCompleted="OutputThumb_DragCompleted"
                       Canvas.Top="-5" PointerEntered="OutputThumb_PointerEntered" PointerExited="OutputThumb_PointerExited"/>
                <TabView x:Name="outputPanelTabView" IsAddTabButtonVisible="False" TabWidthMode="SizeToContent" 
                         VerticalAlignment="Stretch" HorizontalAlignment="Stretch" Height="200">
                    <TabView.TabStripFooter>
                        <Button VerticalAlignment="Center" HorizontalAlignment="Right">
                            <Button.Content>
                                <FontIcon FontFamily="Segoe MDL2 Assets" Glyph="&#xE712;"/>
                            </Button.Content>
                            <Button.Flyout>
                                <Flyout x:Name="outputDockingFlyout" Placement="Bottom">
                                    <Flyout.FlyoutPresenterStyle>
                                        <Style TargetType="FlyoutPresenter">
                                            <Setter Property="CornerRadius" Value="20"/>
                                            <Setter Property="Background" Value="White"/>
                                        </Style>
                                    </Flyout.FlyoutPresenterStyle>
                                    <StackPanel Orientation="Horizontal" Background="White" CornerRadius="20">
                                        <TextBlock Text="Dock Side" HorizontalAlignment="Center" VerticalAlignment="Center"/>
                                        <Button x:Name="outputLeftButton" Background="Transparent" Foreground="Black" BorderBrush="LightGray" 
                                                BorderThickness="1" Margin="5" Padding="0" Width="30" Height="30" Click="OutputLeft_Click">
                                            <Button.Content>
                                                <FontIcon FontFamily="Segoe MDL2 Assets" Glyph="&#xE90C;"/>
                                            </Button.Content>
                                        </Button>
                                        <Button x:Name="outputBottomButton" Background="DeepSkyBlue" Foreground="Black" BorderBrush="DodgerBlue" 
                                                BorderThickness="1" Margin="5" Padding="0" Width="30" Height="30" Click="OutputBottom_Click">
                                            <Button.Content>
                                                <FontIcon FontFamily="Segoe MDL2 Assets" Glyph="&#xE90E;"/>
                                            </Button.Content>
                                        </Button>
                                        <Button x:Name="outputRightButton" Background="Transparent" Foreground="Black" BorderBrush="LightGray" 
                                                BorderThickness="1" Margin="5" Padding="0" Width="30" Height="30" Click="OutputRight_Click">
                                            <Button.Content>
                                                <FontIcon FontFamily="Segoe MDL2 Assets" Glyph="&#xE90D;"/>
                                            </Button.Content>
                                        </Button>
                                    </StackPanel>
                                </Flyout>
                            </Button.Flyout>
                        </Button>
                    </TabView.TabStripFooter>
                    <TabView.TabItems>
                        <TabViewItem Header="Output" IsSelected="True" IsClosable="False" Background="DarkGray" Foreground="White" RightTapped="TabViewItem_Output_RightTapped" >
                            <FlyoutBase.AttachedFlyout>
                                <MenuFlyout>
                                    <MenuFlyoutItem x:Name="FileCopyOutputButton" Text="File Copy" Click="FileCopyOutputButton_Click"/>
                                    <MenuFlyoutItem x:Name="ClipboardCopyOutputButton" Text="Clipboard Copy" Click="ClipboardCopyOutputButton_Click"/>
                                    <MenuFlyoutItem x:Name="ClearOutputButton" Text="Clear" Click="ClearOutputButton_Click"/>
                                </MenuFlyout>
                            </FlyoutBase.AttachedFlyout>
                            <Border Background="LightGray" Margin="5" Padding="0" VerticalAlignment="Stretch" HorizontalAlignment="Stretch">
                                <ScrollViewer VerticalScrollBarVisibility="Visible" VerticalScrollMode="Enabled"
                                              HorizontalScrollBarVisibility="Visible" HorizontalScrollMode="Auto"
                                              VerticalAlignment="Stretch" HorizontalAlignment="Stretch" >
                                    <RichEditBox x:Name="outputText" FlowDirection="LeftToRight" TextAlignment="DetectFromContent" FontFamily="Lucida Sans Unicode,Tahoma" Foreground="Black" Background="AliceBlue" />
                                </ScrollViewer>
                            </Border>
                        </TabViewItem>
                        <TabViewItem Header="Error" IsClosable="False" Background="DarkGray" Foreground="White" RightTapped="TabViewItem_Error_RightTapped" >
                            <FlyoutBase.AttachedFlyout>
                                <MenuFlyout>
                                    <MenuFlyoutItem x:Name="FileCopyErrorButton" Text="File Copy" Click="FileCopyErrorButton_Click"/>
                                    <MenuFlyoutItem x:Name="ClipboardCopyErrorButton" Text="Clipboard Copy" Click="ClipboardCopyErrorButton_Click"/>
                                    <MenuFlyoutItem x:Name="ClearErrorButton" Text="Clear" Click="ClearErrorButton_Click"/>
                                </MenuFlyout>
                            </FlyoutBase.AttachedFlyout>
                            <Border Background="LightGray" Margin="5" Padding="0" VerticalAlignment="Stretch" HorizontalAlignment="Stretch">
                                <ScrollViewer VerticalScrollBarVisibility="Visible"  VerticalScrollMode="Enabled"
                                              HorizontalScrollBarVisibility="Visible" HorizontalScrollMode="Auto"
                                              VerticalAlignment="Stretch" HorizontalAlignment="Stretch" >
                                    <RichEditBox x:Name="errorText" FlowDirection="LeftToRight" TextAlignment="DetectFromContent"  FontFamily="Lucida Sans Unicode,Tahoma" Foreground="Black" Background="FloralWhite" />
                                </ScrollViewer>
                            </Border>
                        </TabViewItem>
                    </TabView.TabItems>
                </TabView>
            </Canvas>
        </Border>

It was working at one point but is no longer and I can't figure out what causes the crash. The panel defaults to showing at the bottom. When I select it to move to either side, the app crashes.