To reproduce:
1) Open some session and open an ui file which would contain a combobox
2) Leave the file open and close Creator
3) Run Creator again and load the same session, previously opened ui file should be visible
4) File | Reload - crash

More info: the ui file need to contain a widget with a "reloadable property sheet", i.e. it shows icons which need to be reloaded, but these icons are not accessible through simple properties.