In main.cpp
MainWindow *w = new MainWindow;
w->show();
In mainwindow.cpp
void MainWindow::on_toolButton_clicked()

{ static TestD *dlg1 = new TestD(this); dlg1->show(); }

void MainWindow::on_toolButton_2_clicked()

{ static TestD *dlg2 = new TestD(this); dlg2->show(); }

if you press both buttons and close the application error message appears
External WM_DESTROY received for QWidgetWindow(0x6b7458, name = "TestDWindow") , parent: QObject(0x0) , transient parent: QWidgetWindow(0x63ac88, name = "MainWindowWindow")