The code looks correct, what issue are you having exactly? Could you post it on imgur so I can take a look? A screenshot or video maybe
Also, what operating system are you testing on?
Viewing post in Sketch (Cartoon) Shader [Post-Processing] comments
Here is a screenshot: https://imgur.com/a/TY6dFkA
I'm using Windows 10 and GameMaker Studio 2
Edit: Actually, maybe this screenshot is more helpful: It is the Main menu and in the background there are background images from other rooms that should not be in the main menu: https://imgur.com/a/mFSqSQI
Ahhh okay, it's not exactly a bug, but the effect is displaying a texture page from the game, because some texture is not associated with the effect. I'm not sure which texture exactly, though... This didn't happen before, so either a GM update did this, or some texture is set incorrectly. Could you check if the textures for the effect are defined correct, and not defined to something like -1?
As far as I can see all textures seem to be defined correctly: https://imgur.com/a/PLQpVf