Scriptable Object resets after Unity is closed

Dear Unity-Comunity,

I have a problem with Scriptable Objects. I generate most of my UI meshes at runtime (like grids, lines, etc.) and I use a Scriptable Object to select the Materials. That makes it a lot easier to quickly change them. My probleme is, that every time I close Unity, the asset (instance of UIData) in my Ressources folder gets reset to these settings:

Best Regards, Théo

My Code:

UIData.cs

 [System.Serializable]
 public class UIData : ScriptableObject {
     [SerializeField]
     public Material Line;
     [SerializeField]
     public float LineThickness;
 
     [SerializeField]
     public Material UnitHighlight;
     [SerializeField]
     public Material WalkArea;
     [SerializeField]
     public Material RunArea;
     [SerializeField]
     public Material Grid;
 }
 

UIDataEditor.cs

 [CustomEditor(typeof(UIData))]
 public class UIDataEditor : Editor {
     private static bool LineFoldout = true;
 
     public override void OnInspectorGUI()
     {
         UIData data = (UIData)target;    
         LineFoldout = EditorGUILayout.Foldout(LineFoldout, "Line");
         if(LineFoldout)
         {
             EditorGUI.indentLevel++;
             data.Line = EditorGUILayout.ObjectField("Line Material", data.Line, typeof(Material), false) as Material;
             data.LineThickness = EditorGUILayout.FloatField("Line Thickness", data.LineThickness);
             EditorGUI.indentLevel--;
         }
         data.UnitHighlight = EditorGUILayout.ObjectField("Unit Highlight Material", data.UnitHighlight, typeof(Material), false) as Material;
         data.Grid = EditorGUILayout.ObjectField("Grid Material", data.Grid, typeof(Material), false) as Material;    
         data.WalkArea = EditorGUILayout.ObjectField("Walk Area Material", data.WalkArea, typeof(Material), false) as Material;
         data.RunArea = EditorGUILayout.ObjectField("Run Area Material", data.RunArea, typeof(Material), false) as Material;
     }
 
     [MenuItem("Assets/Create/UI Data")]
     public static void CreateUIDataAsset()
     {
         Object asset = ScriptableObject.CreateInstance<UIData>();
 
         int i = 0;
         while(true)
         {
             if(AssetDatabase.FindAssets("t:UIData UIData" + i.ToString()).Length == 0)
             {
                 AssetDatabase.CreateAsset(asset, "Assets/UIData" + i.ToString() +".asset");
                 AssetDatabase.SaveAssets();
                 EditorUtility.FocusProjectWindow();
                 Selection.activeObject = asset;
                 break;
             }
             else
             {
                 i++;
             }
         }
     }
 }