MC Selection fails without teams or matches

**Describe the bug**
The MC Alliance Selection script will fail with a printed error if Alliance Selection is attempted to be started before teams are added to the event or any matches are played.

**To Reproduce**
Steps to reproduce the behavior:
1. Create a new event
2. Before adding any teams or matches, start Alliance Selection
3. Attempt to load the MC Alliance Selection script.
4. View the following error (or a similar error when starting with teams, but no matches):
```
TypeError: Cannot read properties of null (reading 'number')
```
**Expected behavior**
Either the attempt to start Alliance Selection or the MC script should fail with a more graceful error message.

**Device (please complete the following information):**
- macOS Venture
- Chrome 117 and Safari 16.6

Reproduce the failure by creating an event with no teams or matches, then starting Alliance Selection and loading the MC Alliance Selection script. Trace the null-value error from the MC script or Alliance Selection startup flow. Done means both empty-event cases fail gracefully with a clear error rather than a TypeError.

Written by the indexing model from the issue text.