#26834: [tk-3dsmaxplus] 3ds Max is crashing if we merge 2 objects

Like the enable/disable menu calls, feels like tracking the current modal dialog should be handled here.  Maybe call the variable "_current_modal_dialog" (or something more specific than "_safe_dialog"?) and also document how to set it to None (or another appropriate value) once the dialog has been closed.

The member variable should be initialized to None in __init__ as well, to make sure that accessing it is safe.