Cannot subscribe to epic notifications

Summary

I'm getting an error when I try to subscribe to notifications of an epic. This is probably a permission error for users outside a group.

Steps to reproduce

Go to any epic of a group you are not a member of (e.g. &31) and try to toggle notifications on.

What is the expected correct behavior?

The notifications toggle button should be either hidden for non-members or allow non-members to subscribe to notifications as in issues or merge requests.

Relevant logs and/or screenshots