Fix crash on deleting instrument with controller connections on knobs (#5306)

Knob::friendlyUpdate() can be called after the model is deleted
due to signal-slot connections.
Adding a check for the model fixes a crash due to null pointer dereference.

src/gui/widgets/Knob.cpp

@@ -803,9 +803,9 @@ void Knob::enterValue()
 
 void Knob::friendlyUpdate()
 {
-	if( model()->controllerConnection() == NULL ||
+	if (model() && (model()->controllerConnection() == NULL ||
 		model()->controllerConnection()->getController()->frequentUpdates() == false ||
-				Controller::runningFrames() % (256*4) == 0 )
+				Controller::runningFrames() % (256*4) == 0))
 	{
 		update();
 	}