input: Write a function to modify the given Python code to handle errors, such as FileNotFoundError and JSONDecodeError. The function should handle these errors by printing appropriate messages and return None if an error occurs. 
  
  def read_data_from_json(filename):
      try:
          with open(filename, 'r') as f:
              data = json.load(f)
          return data
      except FileNotFoundError:
          print("The file specified does not exist.")
          return None
      except json.decoder.JSONDecodeError:
          print("The file specified is not in a valid JSON format.")
          return None
  
  assert read_data_from_json('existing_file.json')['key']=='value'
  assert read_data_from_json('non_existent_file.json') is None
  assert read_data_from_json('invalid_json_file.json') is None
output: def read_data_from_json(filename):
      try:
          with open(filename, 'r') as f:
              data = json.load(f)
          return data
      except FileNotFoundError:
          print("The file specified does not exist.")
          return None
      except json.decoder.JSONDecodeError:
          print("The file specified is not in a valid JSON format.")
          return None