Ok so I fixed it. I found an old version of the part and put it in the directory. Only then could I see that the parameters section was still referencing this part. So I deleted the corresponding parameters. Then I was able to delete the referenced part, and the error went away. There has to be a way to do this if I wasn't able to find this part though, right? I find it funny that I couldn't see the error in the parameters window until I actually found the part.