while sending the document through API I am getting "Object Reference error not set to instance of an object" error

FileStream file = getTestPdfFile(fileName);

            com.echosign.secure.FileInfo[] fileInfos = new com.echosign.secure.FileInfo[1];

            fileInfos[0] = new com.echosign.secure.FileInfo();

            fileInfos[0].fileName = fileName;

            Byte[] bytes = File.ReadAllBytes(".\\" + fileName);

            String filed = Convert.ToBase64String(bytes);

            byte[] filedata = Convert.FromBase64String(filed);

            fileInfos[0].file = filedata;

            SenderInfo senderInfo = new SenderInfo();

            senderInfo = null;

            RecipientInfo[] recipientInfo = new RecipientInfo[2];

            RecipientInfo recipientEmail = new RecipientInfo();

            recipientEmail.email = recipient;

            recipientEmail.fax = null;

            recipientEmail.role = RecipientRole.SIGNER;

            recipientEmail.roleSpecified = true;

            recipientInfo[0] = recipientEmail;

            DocumentCreationInfo documentInfo = new DocumentCreationInfo();

            documentInfo.recipients = recipientInfo;

            documentInfo.name = "Testing Echo";

            documentInfo.fileInfos = fileInfos;

            documentInfo.signatureType = SignatureType.ESIGN;

            documentInfo.signatureFlow = SignatureFlow.SENDER_SIGNATURE_NOT_REQUIRED;

  ES.sendDocument(apiKey, senderInfo, documentInfo);