QmlDesigner: fix crash at shutdown

Found at sentry https://the-qt-company-00.sentry.io/issues/4729592969

Pick-to: qds/4.5
Change-Id: I3e4903eebf5d328361a04f276659bd143eed0925
Reviewed-by: Qt CI Patch Build Bot <ci_patchbuild_bot@qt.io>
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/plugins/qmldesigner/components/texteditor/texteditorview.cpp

@@ -96,6 +96,7 @@ void TextEditorView::modelAboutToBeDetached(Model *model)
 {
     AbstractView::modelAboutToBeDetached(model);
 
+    if (m_widget)
         m_widget->setTextEditor(nullptr);
 
     // in case the user closed it explicit we do not want to do anything with the editor