Show us the wall wart side where it says the voltage that it charges at, it's likely you had something there that acted as a conductor and normally there's not really a risk of shorting. If you want to try again, Clean as much charring away as you can, since it might be a better conductor than the mug material that gap distance was designed for. I would scrape away all the black, use an exacto or razor blade to cut a little more, then fill it with a little bit of 5 minute epoxy or superglue or something.

This is all assuming the voltage and amperage across that gap isn't all that high. Do you think it's likely a drop of water or something got in there as you went to charge it?