fixed
here you are trying to get your capablilty from your backpack w/ no side(null)
https://github.com/Zemelua/UMU-Mod/blob/main/src/main/java/io/github/zemelua/umumod/inventory/container/BelongingsUMUPlayerContainer.java#L61
here you check if there is no side(null)
https://github.com/Zemelua/UMU-Mod/blob/main/src/main/java/io/github/zemelua/umumod/capability/FunctionalBackpackCapabilityProvider.java#L29
so you never get the capablilty
use ifPresent instead of orElse
so .ifPresent(add the slots in here)