Hello, Cookie Badger!
Thank you so much for all the help. You were correct about me using Terrain 3D. I was able to update my Terrain 3D version to the latest (0.9.3). As for the other error, it seemed to just magically go away, as do most random bugs in software development.
This plugin is really good and I'm already using it a lot to design levels. Thank you very much for making it :)
- Nathcra