Repeatable crash that occurs sometimes when a creeper (haven't tested other mobs) dies. Seems to be related to its loot table from what I read in the crash log.

java.lang.NullPointerException: Cannot invoke "java.util.function.Consumer.accept(Object)" because "p_287569_" is null
at net.minecraft.world.level.storage.loot.LootTable.m_287134_(LootTable.java:58) ~[client-1.20.1-20230612.114412-srg.jar%23204!/:?] {re:mixin,re:classloading,pl:mixin:A}
at net.minecraft.world.level.storage.loot.LootTable.handler$zhb000$generateUnprocessedLootMixin_ModifyPopulatedListAndFeedConsumer(LootTable.java:548) ~[client-1.20.1-20230612.114412-srg.jar%23204!/:?] {re:mixin,re:classloading,pl:mixin:A}
at net.minecraft.world.level.storage.loot.LootTable.m_79131_(LootTable.java:93) ~[client-1.20.1-20230612.114412-srg.jar%23204!/:?] {re:mixin,re:classloading,pl:mixin:A}
at reliquary.data.ReliquaryLootModifierProvider$InjectLootModifier.doApply(ReliquaryLootModifierProvider.java:63) ~[reliquary-1.20.1-2.0.45.1248.jar%23200!/:1.20.1-2.0.45.1248] {re:classloading}
at net.minecraftforge.common.loot.LootModifier.apply(LootModifier.java:58) ~[forge-1.20.1-47.3.33-universal.jar%23209!/:?] {re:classloading}

This part specifically.

full crash log: https://pastebin.com/tAfpd5DK

Looks to be issue with Chest Cavity mod, you need to report this to them.