Hi Spencer,
When you activate this function, it defaults to some unknown state. Go ahead an click select to pick the gerber file you want to import. Once you have selected a file click Import to import. The file will import but at the end you will get the error you are mentioning.
The reason for the error is that if you look closely at the dialog on the middle left side you'll see a checkbox that says Run ULP again. Uncheck it. It seems that there is some issue when the ULP tries to call itself again. We'll look into this, but you should be able to run it in the mean time.
Best Regards,

Jorge Garcia
Product Support Specialist for Fusion 360 and EAGLE
Kudos are much appreciated if the information I have shared is helpful to you and/or others.
Did this resolve your issue? Please accept it "As a Solution" so others may benefit from it.