Object reference not set to an instance of an object.

Hi, I'm experiencing the issue that uploading Pdf to the box by API by C#.

While uploading I am getting an unexpected error that Object reference not set to an instance of an object. 

I Want just to upload a Pdf and get a Sharable Link with download and view.

Thank you very much.  

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Here is My Code

 var config = new BoxConfigBuilder(<Client_Id>, <Client_Secret>, new Uri("https://developer.box.com/auth/callback")).Build();
            var client = new BoxClient(config);
            BoxFile newFile;
            using (FileStream stream = new FileStream(@"C:\\Users\\Dhanraj Kumar\\Desktop\\Test.pdf", FileMode.Open))
            {
                BoxFileRequest req = new BoxFileRequest()
                {
                    Name = "Test.pdf",
                    Parent = new BoxRequestEntity() { Id = "0" }
                };
                newFile = await client.FilesManager.UploadAsync(req, stream);
                String Url = newFile.SharedLink.DownloadUrl;
            }