The problem arises when the main window is on secondary display only.

Steps to Reproduce:
1. Compile an run the attached code
2. Install an additional display.
3. Move main window into secondary display.
4. Click "Store" button to save geometry of sub window.
5. Click the "Restore" button to restore geometry of sub window
... and sub window disappears. In fact, it goes out of scope.

This is problem with QWidget::restoreGeometry
To fix this add this:
qwidget.cpp: