fix error in chatbox windows

What does this MR do?

Fixes a sentry error that happens quite often: https://sentry.io/share/issue/388af6964ec040ac8dd2e1926c30696e/

This happens if you close a chatbox while the conversation is still loading.

How confident are you it won't break things if deployed?

Quite sure, works for me

How to test

Steps a reviewer can take to verify that this MR does what it says it does e.g.

1. add a timeout to the loading of a conversation, for example by adding await new Promise((resolve, reject) => setTimeout(resolve, 5000)) before loadConversation
2. Open a chat from the topbar menu
3. Close the chat window before the loader is finished
4. Check the developer console if there is a TypeError: Cannot read property 'el' of undefined

Checklist

• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
• set a "for:" label to indicate who will be affected by this change
• use "state:" labels to track this MR's state until it was beta tested
• added an entry to CHANGELOG.md
• add a short text that can be used in the release notes
• Once your MR has been merged, you are responsible to create a testing issue in Beta Testing Repo:
  • Consider writing a detailed description in German.
  • Describe in a few sentences, what should be tested from a user perspective.
  • Also mention different settings (e.g. different browsers, roles, ...). how this change can be tested.
  • Be aware, that also non technical people should understand.