QmlDesigner: Fix assert

We allow Window, Popup, Dialog etc. for root nodes.

Change-Id: I776c88ab8d276d48f275d842349796c8445259ad
Reviewed-by: Henning Gründl <henning.gruendl@qt.io>
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>

src/plugins/qmldesigner/designercore/model/qmlvisualnode.cpp

@@ -56,6 +56,9 @@ bool QmlVisualNode::isItemOr3DNode(const ModelNode &modelNode)
     if (modelNode.metaInfo().isSubclassOf("QtQuick3D.Node"))
         return true;
 
+    if (modelNode.metaInfo().isGraphicalItem() && modelNode.isRootNode())
+        return true;
+
     return false;
 }