#1245 StoredProjectManager#issueLabelsChanged check if task exists.

[criske]

Fixes #1245

[zoeself]

@criske thank you for your Pull Request. I'll assign someone to review it soon.

[zoeself]

@amihaiemil I couldn't find any assignee for this task. This is either because there are no contributors with role REV available or because the project does not have enough funds.

Please, make sure there is at least one available contributor with the required role and the project can afford to pay them.

[amihaiemil]

@criske thanks, looks good!