I noticed that if you merge on full crates your mod goes in error.
Try to stone fill 5/6 crates and then try to merge them together you will see that it crashes.

Event WideChests::on_player_selected_area
Error while running event WideChests::on_player_selected_area (ID 52)
WideChests/control_init.lua:48: attempt to index field '?' (a nil value)
stack traceback:
WideChests/control_init.lua:48: in function 'move_inventories'
WideChests/scripts/event_handlers/merge_chest.lua:199: in function <WideChests/scripts/event_handlers/merge_chest.lua:185>

Can't reproduce. Can you provide more info? Like mod version, what other mods you have, what (WideChests related) mod setting have non-default value, what type of chest were you trying to merge?