I've seen this crash occur a few times when using the tool to upgrade Assembler 2 entities with zero modules to Assembler 3 with 4 modules.
My guess is that it's trying to insert four modules into the Assembler 2 entity before upgrading the entity.
'''
The mod Blueprint Shotgun (0.2.3) caused a non-recoverable error.
Please report this error to the mod author.
Error while running event blueprint-shotgun::on_tick (ID 0)
Index out of inventory bounds.
stack traceback:
[C]: in function 'index'
__blueprint-shotgun/scripts/build/proxies.lua:130: in function '?'
blueprint-shotgun/scripts/flying-items.lua:21: in function 'on_tick'
blueprint-shotgun/control.lua:199: in function <blueprint-shotgun/control.lua:196>
'''