Error while running event UndergroundIndicatorsFixed::on_player_cursor_stack_changed (ID 33)
UndergroundIndicatorsFixed/control.lua:15: attempt to index field '?' (a nil value)
stack traceback:
UndergroundIndicatorsFixed/control.lua:15: in function 'check_connections'
UndergroundIndicatorsFixed/control.lua:100: in function 'render_for_player'
UndergroundIndicatorsFixed/control.lua:258: in function <UndergroundIndicatorsFixed/control.lua:256>

seems to crash when placing underground belts at all even when using a built underground belt from my inventory crashes before I can even place it.

I was able to fix this by adding a check to the call to check_connections on control.lua line 100:

  if place_result.type == "pipe-to-ground" then
    check_connections(place_result.name)
  end

fixed in latest release