QmlDesigner: Fix recent regression

After calling differenceHandler.typeDiffers() the modeNode
can be invalid. We have to account for this.

Task-number: QDS-2954
Change-Id: I76e28dcd3f1a824802e60bf47b773f5b8cb3d15a
Reviewed-by: Henning Gründl <henning.gruendl@qt.io>
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>

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

@@ -1119,6 +1119,10 @@ void TextToModelMerger::syncNode(ModelNode &modelNode,
         differenceHandler.typeDiffers(isRootNode, modelNode, typeName,
                                       majorVersion, minorVersion,
                                       astNode, context);
+
+        if (!modelNode.isValid())
+            return;
+
         if (!isRootNode && modelNode.majorVersion() != -1 && modelNode.minorVersion() != -1) {
             qWarning() << "Preempting Node sync. Type differs" << modelNode <<
                           modelNode.majorVersion() << modelNode.minorVersion();