I've just downloaded an ISO to check your findings and at some point after navigating to the location of the ISO I see the error that you reported so I can confirm the bug although I'm not sure of how to amend the bug description so that others can reproduce the problem but I was definitely able to start usb-creator more than once earlier.
May be we only see the bug after navigating to the downloaded ISO the first time but usb-creator will always crash afterwards?
Maik, now that is interesting...
I've just downloaded an ISO to check your findings and at some point after navigating to the location of the ISO I see the error that you reported so I can confirm the bug although I'm not sure of how to amend the bug description so that others can reproduce the problem but I was definitely able to start usb-creator more than once earlier.
May be we only see the bug after navigating to the downloaded ISO the first time but usb-creator will always crash afterwards?