QmlDesigner: Set proper ownership on returned C++ object

Change-Id: Id9971cdec65bb84b310196ef84d0bccca9a12e57
Reviewed-by: Qt CI Patch Build Bot <ci_patchbuild_bot@qt.io>
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

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

@@ -274,6 +274,8 @@ PropertyEditorValue *DynamicPropertyRow::createProxyBackendValue()
     auto *newValue = new PropertyEditorValue(this);
     m_proxyBackendValues.append(newValue);
 
+    QQmlEngine::setObjectOwnership(newValue, QJSEngine::CppOwnership);
+
     return newValue;
 }