Fix for 'Crash on exit with tabbed diagrams enabled'

umbrello/uml.cpp
- In function slotTabChanged(int index) / slotTabChanged(QWidget* tab)
  do not call m_doc->changeCurrentView() is s_shuttingDown is true.
  Reason: Calling changeCurrentView accesses objects which have already
  been deconstructed.

BUG: 430322