Crash whem changing state in state:on_finished

I get this error message in red (at the terminal): "Fatal: This state is already stopping: custom"

and this in a small window: "Error: This state is already stopping"

To reproduce this bug write on a custom state:

function state:on_finished(next_state_name, next_state)
  local hero = state:get_game():get_hero()
  hero:start_attack() -- This seems the cause of the Bug
end

and use hero:unfreeze() to start that event. It seems that starting a new state when a custom state is finishing produces the bug.