How do I close the parent form? C # (resolved)

I am used to the flexibility and simplicity of VB6. There are still some inhabits in C. In C #, I created two forms: Form1 and Form2. I added a Button1 in Form1 and added the following code:

Form2 newForm = new Form2 ();

NewForm. Show ();

Then, I added two buttons in Form2. Button1 and Button2. I added the following code in the Click event of Button1:

This. Close ();

The purpose is to disable Form2.

I added the following code in the Click event of Button2:

This. ParentForm. Close ();

My intention is to close its parent form, that is, Form1. However, the following error occurs when this statement is executed:

Why? How can I close the parent form? I hope that you will be able to learn from others.