fix possible crash on workbench change

src/Gui/TaskView/TaskView.cpp

@@ -720,6 +720,19 @@ void TaskView::addTaskWatcher(void)
     updateWatcher();
 
 #if defined (QSINT_ACTIONPANEL)
+#if QT_VERSION >= QT_VERSION_CHECK(5, 12, 0)
+    // Workaround to avoid a crash in Qt. See also
+    // https://forum.freecadweb.org/viewtopic.php?f=8&t=39187
+    //
+    // Notify the button box about a style change so that it can
+    // safely delete the style animation of its push buttons.
+    QDialogButtonBox* box = taskPanel->findChild<QDialogButtonBox*>();
+    if (box) {
+        QEvent event(QEvent::StyleChange);
+        QApplication::sendEvent(box, &event);
+    }
+#endif
+
     taskPanel->setScheme(QSint::FreeCADPanelScheme::defaultScheme());
 #endif
 }