Data inserted and System.NullReferenceException error message on ViewModel class

Dear Herro Wong,

I did what you said but nothing change in my project. When I run as debug mode after I clicked button the code stopped saying System.NullReferenceException error.

I've tried initialize all my properties, class and etc. I removed null value from local parameter even so, when I run the project, I got the same error message.

I don't know that to do next. If do you get some insight about it, please let me know.

Thanks.