[REG 3.1.0.2 -> 3.2.0] Wizard page for configuring file names is broken

1. Start the wizard for a Qt-based project which allows configuring filenames.
   I used the Qt Widgets Application.
2. Proceed to the wizard's last page using defaults.
3. Clear the edit box for the header file's name.
   An error message pops up immediately: "Value does not fall within the expected range."
   That's not exactly helpful when the user wants to type a new name.
4. Close that popup.
   Note that the wizard's "Finish" button is enabled although the header file name is empty.
5. Type a new name for the header file.
6. Clear the edit box for the source file's name.
   Another, different error message pops up: "An error occurred..." Not helpful, either.
7. Close that popup, too.
   Yet another one comes up.
8. Close that popup, too.
   Note that the wizard's "Finish" button still is enabled although the source file name is empty.
9. Click "Finish".
   MSVS crashes and restarts.

All of this worked correctly in snapshot 3.1.0.2.

Found and tested by Squish test tst_new_project_edit.