NullReferenceException: Object reference not set to an instance of an object
at Oxide.Plugins.Backpacks+InventoryWatcher.OnDestroy () [0x0000b] in <39578ff169c54d80949167e7511ac2a8>:0

No problems with backpacks, might be inventory watcher

Not sure what would cause this as it seems like the error can only be triggered if the player object was destroyed in an unusual way, probably by some other plugin. Nevertheless, it's possible to prevent the error and therefore prevent any issues that result from the error (such as the player getting kicked).

I've pushed a change to the plugin's GitHub repo for your testing before the next release on uMod. The error shouldn't happen anymore with this version.
https://github.com/WheteThunger/Backpacks/blob/master/Backpacks.cs