After about 4 hours of gameplay, while I had a depot sitting on a coal patch with 400 drones.

The mod Mining Drones (2.0.2) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event Mining_Drones::on_tick (ID 0)
LuaEntity API call when LuaEntity was invalid.
stack traceback:
[C]: in function 'index'
__Mining_Drones/script/mining_depot.lua:1080: in function 'request_path'
Mining_Drones/script/mining_depot.lua:1023: in function 'process_request_queue'
Mining_Drones/script/mining_depot.lua:1048: in function 'handler'
core/lualib/event_handler.lua:72: in function <core/lualib/event_handler.lua:70>