Tab Sleep Timer Fix oleh Mitch Crane

This is a fix for Tab Sleep Timer by Daniel de Haas which has a bug that causes the add-on to stop working if a timer is ever cancelled. I believe this bug also exists in the Chrome version of the add-on. The original add-on has not been updated for months as of the time I am writing this, so I am assuming it may be abandoned and I decided to offer this fix. If the author fixes his version I will remove this one.