To replicate, place a backpack on the ground, pick it up with either the Portal Gun or Gravity Gun mods, then drop it. This will crash both a single player world and crash a server, and will continue to crash when attempting to load the world back up until uninstalling the mod.

Relevant crashlog is attached, I can grab more if needed

crash-2019-06-20_22.50.26-server.txt

Do you use latest version of the mod?

I believe so, I have the TravellersBackpack-1.12.2-0.1.8a file installed. I also have the latest versions of Gravity Gun, Portal Gun, and iChun Util.

Issue fixed in latest version