Steps to reproduce:

• Choose New/Widget application
• Turn off "Generate form" in Class info page
• Header looks like below:

  #ifndef MAINWINDOW_H
  #define MAINWINDOW_H
  
  #include <QMainWindow>
  
  class MainWindow : public QMainWindow
  {
      Q_OBJECT
  
          public:
                   MainWindow(QWidget *parent = nullptr);
      ~MainWindow();
      
      
      
  };
  #endif // MAINWINDOW_H
  

• Also, it seems that Select All, Ctrl I does not fix it.