Fix for 'Crash double-clicking any knob in debug build (git master) (#948)'

Conflicts:
	src/gui/widgets/knob.cpp

include/JournallingObject.h

@@ -77,6 +77,9 @@ public:
 		return oldJournalling;
 	}
 
+	inline bool isEmpty() {
+		return m_journallingStateStack.isEmpty();
+	}
 
 protected:
 	void changeID( jo_id_t _id );

src/gui/widgets/knob.cpp

@@ -592,7 +592,9 @@ void knob::mouseReleaseEvent( QMouseEvent* event )
 		AutomatableModel *thisModel = model();
 		if( thisModel )
 		{
-			thisModel->restoreJournallingState();
+			if(!thisModel->isEmpty()) {
+				thisModel->restoreJournallingState();
+			}
 		}
 	}