don't try to delete the trashIndex itself

src/core/UBApplication.cpp

@@ -455,6 +455,7 @@ void UBApplication::closing()
     {
         UBDocumentTreeModel *docModel = UBPersistenceManager::persistenceManager()->mDocumentTreeStructureModel;
         documentController->deleteDocumentsInFolderOlderThan(docModel->trashIndex(), UBSettings::settings()->emptyTrashDaysValue->get().toInt());
+        if (docModel->hasChildren(docModel->trashIndex()))
             documentController->deleteEmptyFolders(docModel->trashIndex());
     }