QmlDesigner: fixes crash

If there was no form editor item for the root we crashed in
the drop code

Reviewed-by: Kai Koehne

src/plugins/qmldesigner/components/formeditor/dragtool.cpp

@@ -182,7 +182,7 @@ FormEditorItem* DragTool::calculateContainer(const QPointF &point, FormEditorIte
              return formEditorItem;
     }
 
-    if (scene()->rootFormEditorItem()->boundingRect().adjusted(-100, -100, 100, 100).contains(point))
+    if (scene()->rootFormEditorItem() && scene()->rootFormEditorItem()->boundingRect().adjusted(-100, -100, 100, 100).contains(point))
         return scene()->rootFormEditorItem();
     return 0;
 }