QmlDesigner: Fix crash on removing import

If material shown in material editor is provided by the removed import,
the selected node in material view becomes invalid.

Change-Id: I374e80288b5fe59d4eba63b6712d4c8891fd11cc
Reviewed-by: Mahmoud Badri <mahmoud.badri@qt.io>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>

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

@@ -1917,6 +1917,9 @@ void NodeInstanceView::endNanotrace()
 
 QVariant NodeInstanceView::previewImageDataForGenericNode(const ModelNode &modelNode, const ModelNode &renderNode)
 {
+    if (!modelNode.isValid())
+        return {};
+
     ModelNodePreviewImageData imageData;
 
     // We need puppet to generate the image, which needs to be asynchronous.