Fixes #14

I don't know the used libraries (so I might be wrong in my assumption), but after some debugging, it looks as if the blocking nature of the native alert() function seems to cause the problem in the Promise.rejected function.

If you use for example console.log() the crash does not happen, so I changed the call from alert() to the modal dialogs.alert() function, which works fine for me ... and I checked both cases "update available" and "no update available".