input: Write a function to evaluate a postfix expression. The input to the function should be a string with the postfix expression. The output should be the evaluated value of the expression. If the input expression is invalid, the function should return an error message.
  assert evaluate_postfix("2 3 + 4 5 * *")==46
  assert evaluate_postfix("10 2 8 * + 3 -")==23
  assert evaluate_postfix("5 3 - 2 * 4 +")==6
  assert evaluate_postfix("")=="Invalid expression"
  assert evaluate_postfix("1 2 + *")=="Invalid expression"
output: def evaluate_postfix(expression):
      stack = []
      tokens = expression.split()
      
      for token in tokens:
          if token.isdigit():
              stack.append(int(token))
          else:
              if len(stack) < 2:
                  return "Invalid expression"
              b = stack.pop()
              a = stack.pop()
              if token == '+':
                  stack.append(a + b)
              elif token == '-':
                  stack.append(a - b)
              elif token == '*':
                  stack.append(a * b)
              elif token == '/':
                  stack.append(a / b)
              else:
                  return "Invalid expression"
      
      if len(stack) == 1:
          return stack[0]
      else:
          return "Invalid expression"