Okay.
The application will have a GPL (or EU-PL, haven't decided yet) license, so I uploaded the whole source code to this location.
It is a Qt Creator project file, which I run on the device using MADDE.
It might not be perfect yet, but take into account that this is my first real Qt application.
(And there are really too much TODOs in there.)

I also uploaded a 2 minutes long video file on which you can see the issue. It's here. It's a little blurry, but you can see what I'm talking about. It simply crashes at pressing "Yes" on the message box, or pressing "Done" on the first input dialog. It seems that the second input dialog never crashes, however.
On the video I managed to press the buttons without issue a few times, but sometimes the whole application just crashes for no reason when pressing them.
(And LOL, I accidentally clicked the Weather widget several times after it had crashed...)

The error occours in the MainWindow::addCharacter() method in the mainwindow.cpp.

However, I doubt that the connector or the waitDialog could be the issue, because it is also present when I comment them out.