Did a little debugging. There's a new tile added in 0.18.37 called tile-unknown - likely used by the new blueprinting system to handle modded tiles in blueprints that do not currently exists (due to mod removal). This tile does not have a collision mask, yet all other tiles do. Transport Drones expects it to have one, and is crashing when trying to get the table size of the non-existant table.
This fix works on my end:
Unzip the mod into \mods\Transport_Drones_0.7.9\
in .\script\road_network.lua in the get_tiles function on line 523, add "if tile_mask ~= nil then" after "local tile_mask = tile.collision_mask" then add "end" before "return tiles".
This handles all tiles without collision masks (currently only the new one) and safely allows save game load.
Here's a pastebin snipped of the whole fixed function: https://pastebin.com/rCBCEpzH