[1.20.6] Cancelling MobSpawnEvent.FinalizeSpawn for a spawner causes NullPointerException

[thexaero]

Minecraft Version: 1.20.6

Forge Version: tested on 50.0.9 but doesn't seem to be fixed on 50.0.13

Logs:

java.lang.NullPointerException: Cannot invoke "net.minecraftforge.event.entity.living.MobSpawnEvent$FinalizeSpawn.getDifficulty()" because "event" is null
	at TRANSFORMER/minecraft@1.20.6/net.minecraft.world.level.BaseSpawner.serverTick(BaseSpawner.java:160) ~[forge-1.20.6-50.0.9_mapped_official_1.20.6.jar%230!/:?] {re:classloading}
	at TRANSFORMER/minecraft@1.20.6/net.minecraft.world.level.block.entity.SpawnerBlockEntity.serverTick(SpawnerBlockEntity.java:63) ~[forge-1.20.6-50.0.9_mapped_official_1.20.6.jar%230!/:?] {re:classloading}