input: Write a function to optimize a given objective function using the scipy.optimize.minimize method. The objective function should be defined as 'obj_func(x)' where x is the input variable. The function should return the optimized values of x. If the given input is invalid, the function should return an error message.
  assert optimize() == [optimzed_values]
  assert optimize([invalid_input]) == "Error: Invalid input"
output: def optimize(x=None):
      if x is None:
          return "Error: Invalid input"
      try:
          from scipy.optimize import minimize
          result = minimize(obj_func, x)
          return result.x.tolist()
      except Exception as e:
          return f"Error: {str(e)}"