500 error when selecting the default "Please select..." option from the "Which topic best describes your question?" dropdown

[emilghittasv]

Preconditions

1. Sign in to SUMO
2. Open the browser's network monitor (ctrl + shift + e)

Steps to reproduce
Steps to reproduce the behavior:

1. Go to https://support.allizom.org/en-US/questions/new/firefox/form (or any other freemium product contact form)
2. Select a topic (other than the default value) from the Which topic best describes your question? dropdown menu.
3. Select the default value of the Which topic best describes your question? dropdown menu (Please select...)

Expected behavior
No errors.

Actual behavior
A 500 error message is thrown and sentry fires ValueError Field 'id' expected a number but got ''. -> https://mozilla.sentry.io/issues/6367015720/?project=4504326799753216&query=&referrer=issue-stream

Screencasts

Desktop:

• OS: Windows 11
• Browser: Firefox

Additional context
This issue is reproducible in both stage and prod.

[emilghittasv]

PR mozilla/kitsune#7289