diff --git a/MidiPlayer.Droid/MainActivity.cs b/MidiPlayer.Droid/MainActivity.cs
index 0237270..b43b68e 100644
--- a/MidiPlayer.Droid/MainActivity.cs
+++ b/MidiPlayer.Droid/MainActivity.cs
@@ -45,7 +45,7 @@ public override void OnRequestPermissionsResult(int requestCode, string[] permis
             base.OnRequestPermissionsResult(requestCode, permissions, grantResults);
         }
 
-        protected override void OnCreate(Bundle savedInstanceState) {
+        protected override void OnCreate(Bundle? savedInstanceState) {
             base.OnCreate(savedInstanceState);
             requestPermissions();
             Platform.Init(this, savedInstanceState);
@@ -107,7 +107,7 @@ protected override void OnDestroy() {
             }
         }
 
-        protected override void OnActivityResult(int requestCode, [GeneratedEnum] Result resultCode, Intent data) {
+        protected override void OnActivityResult(int requestCode, [GeneratedEnum] Result resultCode, Intent? data) {
             switch (requestCode) {
                 case (int) Request.SoundFont:
                     _soundFontPath = getActualPathBy(data);
diff --git a/MidiPlayer.FluidSynth/Synth.cs b/MidiPlayer.FluidSynth/Synth.cs
index 9f829b1..fba280e 100644
--- a/MidiPlayer.FluidSynth/Synth.cs
+++ b/MidiPlayer.FluidSynth/Synth.cs
@@ -41,9 +41,9 @@ public class Synth {
 
         static Action<object, PropertyChangedEventArgs> _onUpdated;
 
-        static string _soundFontPath;
+        static string _soundFontPath = string.Empty;
 
-        static string _midiFilePath;
+        static string _midiFilePath = string.Empty;
 
         static SoundFontInfo _soundFontInfo;
 
@@ -439,7 +439,7 @@ public Track(int index) {
             /// <summary>
             /// implementation for INotifyPropertyChanged
             /// </summary>
-            public event PropertyChangedEventHandler PropertyChanged;
+            public event PropertyChangedEventHandler? PropertyChanged;
 
             ///////////////////////////////////////////////////////////////////////////////////////////
             // Properties [noun, noun phrase, adjective]