Attempt to fix competitions

[FireML]

Attempts to fix an issue where competitions are never marked as ended.

• Adds a proper null check in the Competition#handleRewards method, instead of using Objects#requireNonNull
• Puts the competition end code into a try/catch/finally block to ensure the competition is always marked as ended, even if an exception is thrown.

[FireML]

Test build here: https://github.com/FireML/EvenMoreFish/releases/tag/competition-fix