Is your feature request related to a problem? Please describe.
It crashed when we insert stacked liquid bucket of lava/water/etc inside the backpack slot (above tube).

Describe the solution you'd like
Able to fill the bucket slot with stackable filled bucket without crash.

Describe alternatives you've considered
When stackable filled bucket fill the slot, let the fluid fill the tube until full then move the empty bucket below as usual and reduce the above filled bucket according to how much fluid taken to tube.
For now, I just temporally configure the Stacker to hold maximum 1 stack of liquid bucket to avoid crashing.

Additional context
The mods that be able to stack fluid bucket (and other items).
https://www.curseforge.com/minecraft/mc-mods/stacker
Thank you very much sir no hurry!

The crash will be fixed, you still need to put by one bucket though

https://www.curseforge.com/minecraft/mc-mods/travelers-backpack-fabric - Crash fixed, but it still goes by one bucket

forge/fabric?

forge/fabric?
Fabric, 1.18.2. Thank you very much.