Flaky test check_runtimes: [22:54:45]RUNTIME: Cannot read null.assigned_equipment_preset - code/game/objects/items/devices/radio/headset.dm@372

Flaky tests were detected in [this test run](https://github.com/cmss13-devs/cmss13/actions/runs/27919886583/attempts/1). This means that there was a failure that was cleared when the tests were simply restarted.
Failures:
```
check_runtimes: [22:54:45]RUNTIME: Cannot read null.assigned_equipment_preset - code/game/objects/items/devices/radio/headset.dm@372 at code/defines/../modules/unit_tests/check_runtimes.dm:10
```

Start with code/game/objects/items/devices/radio/headset.dm at line 372 and the check_runtimes test at code/defines/../modules/unit_tests/check_runtimes.dm:10. Reproduce the failure through the linked GitHub Actions test run or a restart, then trace why assigned_equipment_preset is read from null. Done means the runtime no longer occurs and check_runtimes passes reliably.

Written by the indexing model from the issue text.