…#53553)

When `FlutterUndoManagerPlugin` deallocated, it is supposed to deregister itself from the `NSUndoManager` stack.
However, during dealloc `_undoManagerDelegate.undoManager` is nil (`_undoManagerDelegate` = the engine, and `undoManager` was its view controller's `undoManager`, which was already gone).

Since `_undoManagerDelegate.undoManager` is nil, it doesn't actually call `-[NSUndoManager removeAllActionsWithTarget]`. In the add-to-app scenario, after the view controller pops back to the native view, and "undo" is invoked, the undo action for the `FlutterUndoManagerPlugin` is handled, but it already dealloced so crash.

Fixes flutter/flutter#150408

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

…... (#53575)

If this roll has caused a breakage, revert this CL and stop the roller
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