TypeError if subscriber gets deleted before notifications are sent

Problem/Motivation

The processing of the notify subscriber queue throws a type error if the subscriber unsubscribes before the queue item in the notify subscriber queue gets processed.

Steps to reproduce

• Subscribe to an entity
• The entity gets updated with notifications enabled
• The subscriber unsubscribes from all entities or gets deleted
• The queue processing is triggered
• A type error (not an exception) is thrown and the item gets stuck in the queue

Proposed resolution

• Check if subscriber or subscriber->entity are empty in processItem of QueueWorker and if so, return silently

Remaining tasks

User interface changes

API changes

Data model changes