TypeError：“NoneType”类型的参数对于作为参数传递给 function 的列表不可迭代

Question

Given an array nums of distinct integers, return all the possible permutations. You can return the answer in any order.

I keep getting the error, and I don't understand why.

TypeError: argument of type 'NoneType' is not iterable
    if (nums[i] not in current):

This is my code:

def permute(self, nums: List[int]) -> List[List[int]]:
        output = []
        self.backtracking(nums, output, [],0, len(nums))
        return output 
        
    
    def backtracking(self, nums, outputList, current, length, maxLength):
        if (length == maxLength):
            outputList.append(current)
        
        for i in nums:
            if (nums[i] not in current):
                self.backtracking(nums, outputList, current.append(nums[i]), length+1, maxLength)