Game Maker Text Box Engine Oil

Problem with textbox engine

I'm using the textbox system that created HeartBeast in this tutorial (https://www.youtube.com/watch?v=b1hT-nzIds4), but I have a few issues trying to destroy the textbox when all the text is displayed on screen.

In his tutorial, the text will be scrolling down when the text don't fit on the texbox. I've edited the engine to have manual scroll. But my problem isn't here. I don't find a clue how to delete this textbox only when all the text have been displayed on screen and press 'A'.

This is the Draw event.

Any clue on how to work with that? Maybe I'm complicating it too much and there is a good textbox system created elsewhere.