TypeError when submitting a flag with custom content (require_message flags)

[tgxworld (Alan Tan)]

@cuo_wu

Steps to Reproduce

1. Visit any topic post
2. Click the flag button to open the flag modal
3. Select a flag type that requires a message (e.g. “Something Else” / notify_moderators, or any custom flag created in Admin → Flags with “Require message” enabled)
4. Type a message in the text area (long enough to pass the minimum length)
5. Click the submit button

I tried to reproduce locally but am unsuccessful. Are you able to reproduce this if you enable safe-mode?