SegFault by uploading a score and quickly closing the score before the confirmation popup.

Without looking at the code, I can guess that what is happening is that when the confirmation from musescore is received, it uses a pointer to the score object to recall which score to update the score's url for, without bothering to check if that score object is still valid. So maybe check through the list of open scores and only update the score if can find an open score matching the pointer.