Problem to show alert message after upgrade of xamarin forms 2.0 for android.

Hi Guys,
After upgrade of xamarin forms 2.0 for android, I am getting the problem to show alert message. I am using UserDialogs.Instance.Alert("Error"); to show the alert message. When debugger comes to this point, the application breaks and it show me the error java.lang.NullPointerException. I have attached the screenshot of output window.

Thank you.