ModelEditor: Avoid corrupt diagrams after moving end of relation

Change-Id: If0a2a62d3ef7e59242b244d6892e83bb71315164
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/libs/modelinglib/qmt/diagram_controller/diagramcontroller.h

@@ -87,6 +87,7 @@ public:
     void setModelController(ModelController *modelController);
     UndoController *undoController() const { return m_undoController; }
     void setUndoController(UndoController *undoController);
+    QList<MDiagram *> allDiagrams() const { return m_allDiagrams; }
 
 private:
     MDiagram *findDiagram(const Uid &diagramKey) const;