Setting an object's material causes strange behavior in custom editor

I have an editor script which is supposed to allow me to change the texture of a tile (I'm making a game board). It works by creating a drop down menu with different "tile types", and then when I select a new tile type from the menu it will change the texture to the appropriate type.

 [CustomEditor(typeof(Tile))]
 public class TileEditor : Editor {
 
     enum Type {
         Blank,
         Portal
     }
 
     // Represents the type of tile this is
     Type type;
 
     public override void OnInspectorGUI() {
         Tile tile = (Tile)target;
 
         // Create a dropdown list of tile types
         type = (Type)EditorGUILayout.EnumPopup ("Type", type);
 
         switch (type) {
         case Type.Blank:
             tile.GetComponent<Renderer> ().material = Resources.Load ("Materials/Blank Tile", typeof(Material)) as Material;
             break;
         case Type.Portal:
             tile.GetComponent<Renderer> ().material = Resources.Load("Materials/Portal Tile", typeof(Material)) as Material;
             break;
         }
     }
 }

If I remove the lines where I set the materials, the drop down menu works as expected, but with those lines included, when I select Portal from the drop down it will change the texture for just a moment, and then both the texture and the menu will reset themselves to Blank.

What is going on here, and how do I make my selections persist?