gdb does not show elements of MainWindow's ui

1. Create a new Qt Widgets Application using a MinGW-based kit.
   I used Qt 5.12.1.
2. Open the project's file "mainwindow.ui"
3. In Design mode, place a QPushButton in the MainWindow.
4. Right-click the QPushButton and select "Go to slot..." from the context menu.
5. Select "clicked()" and click "OK".
6. Add some code and a breakpoint into the slot.
7. Start debugging the application and click the QPushButton.
   The breakpoint is being hit.
8. In the Locals view, expand "this".
   One of it's members is "ui".
9. Try to expand "ui".
   This does not work. Only the Value and Type of the pointer are shown.

It should be possible to list the members of the object pointed to by expanding it. With cdb, this seems to work.