inGameAdBtn = GetComponent<Button>();

This line of code says "throw away whatever the designer assigned to the `inGameAdBtn` in the Inspector, and replace it with the `Button` component attached to this object.

...but there is no `Button` component attached to this object in your screenshot. So this throws away a perfectly good button and replaces it with `null`, the absence of a button.

Naturally when you next try to use this button, while holding only the absence of a button, you get an exception.

If you plan to wire up this reference in the Inspector, delete this line of code that tries to search for it on this object — it's not doing anything you want it to do.