QmlDesigner: Fix crash

The timer can timeout after a detach.

Change-Id: I7eafe0a3d666942841cc0ee64d850acfab418c7c
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

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

@@ -593,10 +593,12 @@ void PropertyEditorView::modelAttached(Model *model)
     m_locked = true;
 
     if (!m_setupCompleted) {
-        QTimer::singleShot(50, this, [this]{
-            PropertyEditorView::setupPanes();
-            /* workaround for QTBUG-75847 */
-            reloadQml();
+        QTimer::singleShot(50, this, [this] {
+            if (isAttached()) {
+                PropertyEditorView::setupPanes();
+                /* workaround for QTBUG-75847 */
+                reloadQml();
+            }
         });
     }