input: Write a function to complete the Python class below so that it can calculate the sum of two integers. If the input is an empty class or does not have two integers to sum, return an appropriate error message.
  
  class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
          
      def add(self):
          if self.x is None or self.y is None:
              return "Error: Both inputs must be integers."
          if not isinstance(self.x, int) or not isinstance(self.y, int):
              return "Error: Both inputs must be integers."
          return self.x + self.y
  
  assert Sum(5, 3).add() == 8
  assert Sum(10, -10).add() == 0
  assert Sum(15, "a").add() == "Error: Both inputs must be integers."
  assert Sum(None, 4).add() == "Error: Both inputs must be integers."
  assert Sum(3, None).add() == "Error: Both inputs must be integers."
output: def add(self):
      if self.x is None or self.y is None:
          return "Error: Both inputs must be integers."
      if not isinstance(self.x, int) or not isinstance(self.y, int):
          return "Error: Both inputs must be integers."
      return self.x + self.y