Backport fix made in commit 5073, 5077, 5079 in branch 0.6

git-svn-id: svn+ssh://svn.tuxfamily.org/svnroot/qet/qet/branches/0.60@5085 bfdf4180-ca20-0410-9c96-a3a8aa849046

sources/editor/elementscene.cpp

@@ -48,6 +48,14 @@ ElementScene::ElementScene(QETElementEditor *editor, QObject *parent) :
 	m_qgi_manager(this),
 	m_element_editor(editor)
 {
+	
+	setItemIndexMethod(QGraphicsScene::NoIndex);
+		//Set to no index, because they can be the source of the crash with conductor and shape ghost.
+		//https://forum.qt.io/topic/71316/qgraphicsscenefinditembsptreevisitor-visit-crashes-due-to-an-obsolete-paintevent-after-qgraphicsscene-removeitem
+		//https://stackoverflow.com/questions/38458830/crash-after-qgraphicssceneremoveitem-with-custom-item-class
+		//http://www.qtcentre.org/archive/index.php/t-33730.html
+		//http://tech-artists.org/t/qt-properly-removing-qgraphicitems/3063
+	
 	m_behavior = Normal;
 	setItemIndexMethod(NoIndex);
 	setGrid(1, 1);