Cannot free all Qt MainWindow plugin when call delete instance form main thread

Cannot free all Qt MainWindow plugin when call delete instance form main
thread

I have the Qt plugin, it is implemented inherit QMainWindow. In main
thread, i have code: ...
QDir plugDir(pluginsDir);
#ifdef _DEBUG
QString filename = plugDir.path() + "/" +
"FormDefinitionPluginD.dll";
#else
QString filename = plugDir.path() + "/" + "FormDefinitionPlugin.dll";