My version of Blender (3.3.1) doesn’t pop up with a Viewer Node when I follow Grant’s hotkey. Instead, I added a regular Emission node and used that to try and bake the Emission Cavity texture. But then I was only getting black textures are a result! I banged my head on this issue for awhile until I thought to maybe just save the black texture as it was and then restart Blender. And sure enough, that worked, and next try I had a successful bake.
Just leaving this tip here as a breadcrumb to future learners so you won’t have to waste precious minutes trying to figure this out like me. 