input: Write a function to modify the given Python code to handle errors, including handling the case where the specified file does not exist and the case where the file is not in a valid JSON format. 
  
  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.")
      except json.decoder.JSONDecodeError:
          print("The file specified is not in a valid JSON format.")
  
  assert read_data_from_json('nonexistent.json')==None
  assert read_data_from_json('invalid_format.json')==None
  assert read_data_from_json('valid.json')=={'key': 'value'}
output: def read_data_from_json(filename):
      try:
          with open(filename, 'r') as f:
              data = json.load(f)
          return data
      except FileNotFoundError:
          return None
      except json.decoder.JSONDecodeError:
          return None