Close plot window when corresponding MDHisto workspace was deleted

[SilkeSchomann]

Describe the bug
After deleting a workspace from the MDHisto tab while the corresponding plot window is still open, a change to the intensity causes a crash as the underlying workspace is not available anymore.

To Reproduce
Steps to reproduce the behavior:

1. Load a workspace into MSlice
2. Do a cut
3. Select the corresponding workspace in MDHisto and click the Delete button
4. Change the intensity in the plot window
5. Mantid error

Expected behavior
The plot window should get closed when the workspace is deleted in the MDHisto tab.