Found a workaround. If you go to the project folder you will notice that you do have the .design folder, but it is missing the .csproj file. If you can get a fresh design.csproj you can just put in in there and rename it as needed. If you need that, check this thread:
You’ll need to right click on the ProjectName folder in the solution explorer, add project, and choose the .csproj file. Edit the file to match your project. From there on you can use the activity creator as normal.
When you build the .nupkg, I found that I needed to do this:
The Activity Creator does not create package - Help / Studio - UiPath Community Forum
And use the Build function instead of Publish.
Also, at some point it said I was missing a packageIcon.png file to create the .nupkg, so I just added some random .png to the folder and it seems to have worked.