Attempt to insert 9Slice RenderData twice warning in erroneous places

I have a script that counts the number of items in a resource folder and gives that number to a TextMeshPro text field. That's all it does.

Code (CSharp):

1.      [SerializeField]
2.      string folderToLoadFrom = "hats";
3.      // Use this for initialization
4.      void Start () {
5.          GetComponent<TextMeshProUGUI>().text = Resources.LoadAll(folderToLoadFrom + "/").Length.ToString();
6.      }

For whatever reason this throws a warning: Attempt to insert 9Slice RenderData twice.

This isn't the only place I get this warning popping up. I also get it when I add a SpriteRenderer component to a gameobject that is created via script. No idea why it's showing up or what it even means.

Any help?

 

I'm getting this too. Never got it before. It must have something to do with a recent update. I get it when I try to instantiate a certain GameObject.

EDIT: This seems to be a problem only in the edtor in 2018.3.1f1