Just got this on placing drone ...
The mod Cargo Drone (1.9.1) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event cargo-drone::on_tick (ID 0)
cargo-drone/scripts/depot_helper.lua:67: attempt to index field 'depot_helper' (a nil value)
stack traceback:
cargo-drone/scripts/depot_helper.lua:67: in function 'get_depots'
cargo-drone/scripts/scheduler.lua:337: in function 'assign_depot_task'
cargo-drone/scripts/scheduler.lua:551: in function '?'
cargo-drone/scripts/scheduler.lua:574: in function 'tick'
cargo-drone/control.lua:57: in function <cargo-drone/control.lua:54>

Thank you for the report!

Hmm... I've been unable to reproduce the error so far
Is it a new save, did you use a save from a previous version of the mod, or did you add the mod to an existing save?
Could I also have the log file from when the error happened, or any lines inside the log file with cargo-drone in it? You can find the log file here %appdata%/Factorio/factorio-current.log

Never mind, I've managed to reproduce it
Using a save from a previous mod version (pre-1.8.0) with no moorings placed in the world. Then running the save with the current version will cause Cargo drones to create the error
This is due to a code mistake making the mod in the save enter an invalid state during migration. Should be a simple fix

Fixed in 1.9.2