Undo/redo history malfunction

[atasman]

There are several issues with the redo and undo history.
If you've got a state with an extracted subdiagram opened in an other editor tab and try to remove the state, a dialog will asks you whether the subdiagram should be closed or not. If you choose to keep the subdiagram, multiple exceptions are thrown having their cause in the undo methods of undoable operations .

Another issue can be reproduced by doing the following steps:

1. Open two different statecharts (statechart1, statechart2)
2. Make a change in statechart1
   [2.1 Undo change via menu entry "Undo [...]" ]
3. Close statechart2

Doing this will keep the menu entry "Undo [...]" or "Redo [...]" enabled without any functionalities.