input: Write a python function to simulate a simple dialogue system that responds to 'Hi' with 'Hello there!' using Flask.
  assert dialog_response('Hi') == 'Hello there!'
  assert dialog_response('Hello') == None
  assert dialog_response('Goodbye') == None
output: from flask import Flask, request, jsonify
  
  app = Flask(__name__)
  
  @app.route('/dialog', methods=['POST'])
  def dialog_response():
      data = request.get_json()
      message = data.get('message')
      if message == 'Hi':
          return jsonify(response='Hello there!')
      return jsonify(response=None)
  
  if __name__ == '__main__':
      app.run(debug=True)