How to invoke without parameters method?

I have one class in which public method without input parameter.

 public partial class MyClass: System.Web.UI.MasterPage
   {

      public void HelloWorld() { 
       Console.WriteLine("Hello World "); 
      } 
    }

I want to invoke HelloWorld() method into my another class

public partial class ProductType_Showpt : System.Web.UI.Page
{
     protected void ChkChanged_Click(object sender, EventArgs e)
    {
          MyClass master =(MyClass) this.Master;   
          master.GetType().GetMethod("HelloWorld").Invoke(null, null);
    }
}

but it's throw this exception

Object reference not set to an instance of an object.