Werk #6418: Fix a crash during notification cancelling

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.

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