Implement Masapayment With payapal gives error -object ref

Question

I am working with c# implementing Mass payment with paypal
here is my code but i am getting error on calling api
("Object reference not set to an instance of an object.") 
 at line
 responseMassPayResponseType = service.MassPay(massPay);

Please help me to sort out this.
Please check the attachment my class which i am using


Thanks in advance