Naturally, the engine checks scripts before setting up autoloads (especially for the autoload script itself), so it cannot find the TypeHandler during parsing and gives an error. In which file did you use this line?
I have a custom resource file called basic_monster.gd that I use it in, but I actually use this in quite a few places. This seems to be the only one that has an error. I just don’t understand why it would randomly start doing this because it has never been an issue before.