Score state corruption and crash after adding a frame to a nested horizontal frame
1) Add a vertical frame somewhere in a score.
2) Right-click the frame, choose Add→Add Horizontal Frame.
3) In Palettes click on "Add Vertical Frame" or "Add Horizontal Frame".
Result: everything disappears, only a few frames are displayed.
4) Undo 3 times.
Result: crash. Also neither of "undo" actions doesn't restore to displaying anything meaningful in a score.
Actually there doesn't seem to be any sense in doing such things with nested frames, but this is still a working way to inadvertently break MuseScore.