Communication: Conversation does not load after deleting forwarded source post

[julian-wls]

Describe the bug

While working on this PR the following problem has been identified:
Whenever a source post, that has previously been forwarded, is deleted it is not possible anymore to navigate to the conversation that contains the destination post. When trying to navigate to the conversation the posts are not loaded correctly and the posts of the previously selected conversation are still shown.

To Reproduce

1. Go to a conversation
2. Forward a post to another conversation
3. Delete the source post
4. Try to navigate to the other conversation
5. Verify that the posts of this conversation are not loaded correctly

Expected behavior

The correct posts should be loaded.

Screenshots

Bug.mp4

Which version of Artemis are you seeing the problem on?

7.10.3 and 8.0.0

What browsers are you seeing the problem on?

Chrome

Additional context

This problem was mainly observed on larger conversations. Sometimes, when testing on conversations that only included a small number of posts, the problem could not be reproduced.
This Problem was discovered while testing this PR but has been confirmed by multiple developers to be reproducible on other branches.

Relevant log output