QmlDesigner: Fix soft assert

The node can acutally be invalid.

Change-Id: I18430a538ce947c724ddca85e9fc5e40175019d4
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>
Reviewed-by: Henning Gründl <henning.gruendl@qt.io>

src/plugins/qmldesigner/components/propertyeditor/propertyeditorqmlbackend.cpp

@@ -481,7 +481,9 @@ static NodeMetaInfo findCommonSuperClass(const NodeMetaInfo &first, const NodeMe
 
 NodeMetaInfo PropertyEditorQmlBackend::findCommonAncestor(const ModelNode &node)
 {
-    QTC_ASSERT(node.isValid(), return {});
+    if (!node.isValid())
+        return {};
+
     QTC_ASSERT(node.metaInfo().isValid(), return {});
 
     AbstractView *view = node.view();