Hello!
There was a regular iron chest with items inside, I replaced the chest with a request chest (blue), and an error appeared.

Thanks for the report. I will look into it.

Oops, sorry, forgot to include the error description. The error was also reproduced.
Correction:

Error while running event inventory-sensor-improved::on_tick (ID 0)
LuaEntity API call when LuaEntity was invalid.
stack traceback:
[C]: in function 'index'
__inventory-sensor-improved/scripts/sensor.lua:323: in function 'connect'
inventory-sensor-improved/scripts/sensor.lua:159: in function 'scan'
inventory-sensor-improved/scripts/sensor.lua:390: in function 'tick'
inventory-sensor-improved/scripts/event-setup.lua:188: in function 'handler'
inventory-sensor-improved/stdlib/event/event.lua:344: in function 'pcall'
inventory-sensor-improved/stdlib/event/event.lua:364: in function 'dispatch_event'
inventory-sensor-improved/stdlib/event/event.lua:440: in function <inventory-sensor-improved/stdlib/event/event.lua:398>

This is fixed in 1.2.1. I missed the one spot for a "valid" check ...