QmlDesigner: Fix assert

If in a reaction to auxiliaryDataChanged() we change AuxiliaryData
this could trigger cascades and to avoid this we assert.

The cascades and assert can be avoided by checking AuxiliaryData
before.

Change-Id: Id9dd4198041889113fb0e50fdfd4785fddf89f02
Reviewed-by: Mahmoud Badri <mahmoud.badri@qt.io>

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

@@ -661,6 +661,9 @@ void FormEditorView::toggle3DViewEnabled(bool enabled)
     QTC_ASSERT(model(), return);
     QTC_ASSERT(rootModelNode().isValid(), return);
 
+    if (enabled == rootModelNode().hasAuxiliaryData("3d-view"))
+        return;
+
     if (enabled)
         rootModelNode().setAuxiliaryData("3d-view", true);
     else