To be honest, I don't remember what I wrote there :P
Your error means that the surface wasn't created yet (somewhere You should have one line of code like surface = surface_create() before making any changes on it).
Are you working on this in your own project or in my demo? Because if you're running the demo and getting this error, it might mean your version of Game Maker is too new and it's not compatible - so i have to repair it in short future.