Error when attempting to promote an issue

Summary

The /promote quick action raises an exception on the backend for some issues. See the Sentry report. (Also, the error message displayed in the UI is quite misleading, but that is being addressed under #21307).

Steps to reproduce

1. Find an issue with an associated ResourceMilestoneEvent which has no associated milestone
2. Submit a comment of /promote
3. Encounter this error message: "Your comment could not be submitted! Please check your network connection and try again."

What is the current bug behavior?

Using /promote in certain issues does not promote the issue, but does cause an error message to be displayed.

What is the expected correct behavior?

Using `/promote in these issues causes them to be promoted regardless of the milestone assignment.

Relevant logs and/or screenshots

Output of checks

This bug happens on GitLab.com.