input: Write a python function to generate a bot response for a user message using random choice.
  assert generate_bot_response('hello') in ['Hi', 'Hi there', 'Hello, how are you?', 'Hey there']
  assert generate_bot_response('exit') == None
output: import random
  
  def generate_bot_response(user_message):
      responses = {
          'hello': ['Hi', 'Hi there', 'Hello, how are you?', 'Hey there'],
          'exit': None
      }
      
      if user_message in responses:
          if responses[user_message] is None:
              return None
          return random.choice(responses[user_message])
      return "I'm not sure how to respond to that."