fix crash at session close/exit if a midi-control-surface is used

fix crash at session close/exit if a midi-control-surface is used

The "real" problem was that MidiControlUI is destroyed
while there are still MIDIControllables around that
still have a signal connection to a Controllable:
  controllable->Destroyed(..., MidiControlUI::instance())
If a Contrallable is deleted after the MidiControlUI
event loop is gone, the Destroyed() signal can create odd
situations...