QmlDesigner: Fix a rare crash

Lack of model existence check causes crash in some situations where
there are other problems in the document.

Change-Id: I239fc55de50e4795a97c60add3c820c6f2c6f73c
Reviewed-by: Mahmoud Badri <mahmoud.badri@qt.io>

src/plugins/qmldesigner/designercore/instances/nodeinstanceview.cpp

@@ -1860,6 +1860,9 @@ void NodeInstanceView::updateWatcher(const QString &path)
 
 void NodeInstanceView::updateRotationBlocks()
 {
+    if (!model())
+        return;
+
     QList<ModelNode> qml3DNodes;
     QSet<ModelNode> rotationKeyframeTargets;
     bool groupsResolved = false;