Widgets/Knob: only restore journalling state on release of left mouse button

Do not try to restore the journalling state of the underlying model when
releasing e.g the right mouse button as this causes a crash otherwise.

src/gui/widgets/knob.cpp

@@ -488,9 +488,16 @@ void knob::mouseMoveEvent( QMouseEvent * _me )
 
 
 
-void knob::mouseReleaseEvent( QMouseEvent * /* _me*/ )
+void knob::mouseReleaseEvent( QMouseEvent* event )
 {
-	model()->restoreJournallingState();
+	if( event && event->button() == Qt::LeftButton )
+	{
+		AutomatableModel *thisModel = model();
+		if( thisModel )
+		{
+			thisModel->restoreJournallingState();
+		}
+	}
 
 	m_buttonPressed = false;