Check_MK Werk 6418: Fix a crash during notification cancelling

ID: 6418 Title: Fix a crash during notification cancelling Component: Notifications Level: 1 Class: Bug fix Version: 1.6.0i1 When a notification was cancelled and the cancelling rule contained more contacts than the notification that was going to be cancelled the processing of notifications raised an exception. As a result no further notifications were processed. The exception was raised e.g. if a notification for the contact "ab" was cancelled for the users "ab" and "cd" using the option "The following users" in the contact selection. With this werk the issue is fixed.