[BUG] Client never starts game when client ids are not within 1 to 10

[mFragaBA]

🐛 Bug Report

The game doesn't start for the client if we run load tests and a client at the same time. This is because there's one function where we're assuming that client IDs go from 1 to 10 and this might not necessarily true (in fact in other places we don't).

(Write your description here)

Steps to Reproduce

1. start the backend application
2. start the load test application
3. run the game on unity and start a new game
4. before the game starts, start a load test run with LoadTest.PlayerSupervisor.spawn_players(50, 100)

Stack trace & error message

NullReferenceException: Object reference not set to an instance of an object
CustomLevelManager+<>c__DisplayClass35_0.<GetCharacterPrefab>b__0 (CoMCharacter el) (at Assets/Scripts/CustomLevelManager.cs:162)
System.Collections.Generic.List`1[T].Find (System.Predicate`1[T] match) (at <492e64214f0c42f2b51bf2d39219ebe6>:0)
CustomLevelManager.GetCharacterPrefab (System.UInt64 playerId) (at Assets/Scripts/CustomLevelManager.cs:162)

Screenshot

Expected Behavior

The game should start regardless of the player ids

Your Environment

Tested on branch main.
Tested on MacOS 13.5.1
Tested on Unity 2021.3.26.3f