input: Write a python function to advance to the next level in a text adventure game and handle the level transitions.
  assert advance_to_next_level(level_1) == level_2
  assert advance_to_next_level(level_2) == level_3
  assert advance_to_next_level(level_3) is None
output: def advance_to_next_level(current_level):
      level_transitions = {
          'level_1': 'level_2',
          'level_2': 'level_3',
          'level_3': None
      }
      return level_transitions.get(current_level)