ERT ghost button disappearing

### Testmerges

#10999: 'Fix of various plane related issues' by Blundir at commit f85a7153c4 #11075: 'Ticket Panel (Adminhelps & Mentorhelps)' by GeneriedJenelle at commit 4ad8d0cf87 #11902: 'implement pii scrubbing for sentry' by hry-gh at commit 759efb7bd1 #11170: 'Yautja Survivor' by realforest2001 at commit 83a03ddd01 #11724: 'Point Blank Refactor [EXPERIMENTAL]' by TrollerNoob at commit e1277b8075 #10925: 'Cape River Mining Colony - Ground Map' by Steelpoint at commit 24b927ce86

### Round ID

33570

### Description of the bug

If two ERTs are active for people to join at the same time (in this case it was the WY chem team ERT followed by foxtrot spec ERT) when the first ERT closes applications, the UI button near top left to join ERT disappears.

### What's the difference with what should have happened?

The button shouldn't disappear if another ERT is still actively taking applications.

### How do we reproduce this bug?

Trigger two ERTs close together, watch as button disappears when first ERT closes applications but second team hasn't closed them yet.

### Issue Bingo

- [ ] Issue could be reproduced at least once
- [x] Issue happened in a recent (less than 7 days ago) round
- [x] Couldn't find an existing issue about this (https://github.com/cmss13-devs/cmss13/issues)

No files, tests, or entry points are named. Reproduce the issue by starting two ERT applications close together, then observe the top-left join button as the first application closes; done means the button remains available while another ERT is still accepting applications.

Written by the indexing model from the issue text.