c#：运行中报错System.ArgumentNullException: 'Value cannot be null. Parameter name: json'

代码：

public async void GetDoc()
        {
            HttpClient client = new HttpClient();
            var randDoc = await mfunsAPI.GetRandDoc(10);
            StackLayout stack = Content as StackLayout;
            for (int i = 0; i < randDoc.Length; i++)
            {
                var Doc = JsonDocument.Parse(randDoc[i]).RootElement;
                Button button = new Button { Text = Doc.GetProperty("id").GetInt32().ToString()};
                int id = Doc.GetProperty("id").GetInt32();
                button.Clicked += async (sender, args) => await GoToDoc(id);
                stack.Children.Add(button);
                Content = stack;
            }
            //上面那个for循环结束报错
        }

所有代码里都没有出现过json这个变量，太奇怪了