FdoServer: drop bubble reference when `destroy()` is emitted

[Marukesu]

our bubble map was keeping a reference to the bubble, stoping it from being destroyed. drop our reference when destroy() is emitted.

Fixes: #212.

[leolost2605]

Thanks for the quick response :) I think with the updates to #211 we won't need this anymore but if you want to keep it as a failsave for future errors I'm happy to approve

[Marukesu]

i perfer to revert #211 back to the approved state, the changes done there will make close_notification() throw for notifications in the tray.

[leolost2605]

Oh man I'm starting to loose track 😅. I reverted the changes there

[Marukesu]

Good catch! Added the null check.