任何人都可以纠正问题填写在GridView的数据

我写了一个查询。

SqlDataAdapter da = new SqlDataAdapter("select users.username,groups.name from users,groups inner join users on roles.userid=users.userid inner join groups on roles.groupid=users.groupid", MAconn); 

but i am getting an error

 

 

System.NullReferenceException: Object reference not set to an instance of an object. at Default2.bind()