add unlink possibility to IView

GUI/Views/SampleDesigner/IView.cpp

@@ -24,8 +24,7 @@ IView::IView(QGraphicsItem* parent) : QGraphicsObject(parent), m_item(nullptr)
 
 IView::~IView()
 {
-    if (m_item)
-        m_item->mapper()->unsubscribe(this);
+    unlinkFromItem();
 }
 
 void IView::setParameterizedItem(SessionGraphicsItem* item)
@@ -52,6 +51,13 @@ void IView::setParameterizedItem(SessionGraphicsItem* item)
 
 void IView::addView(IView*, int) {}
 
+void IView::unlinkFromItem()
+{
+    if (m_item)
+        m_item->mapper()->unsubscribe(this);
+    m_item = nullptr;
+}
+
 void IView::onChangedX()
 {
     if (!m_item)

GUI/Views/SampleDesigner/IView.h

@@ -36,6 +36,10 @@ public:
 
     virtual void addView(IView* childView, int row = 0);
 
+    //! Call this in case you call deleteLater() for the view and the
+    //! linked item itself may not exist any more at the time of deletion
+    void unlinkFromItem();
+
 signals:
     void aboutToBeDeleted();