Texture2D variable causes MissingReferenceException

I've suffered a MissReferenceException when using Texture2D. With below code,

 public abstract class TestBase
 {
     protected readonly Texture2D texture;
     
     public TestBase()
     {
         texture = new Texture2D(100, 100, TextureFormat.RGB24, false);
     }
     
     public abstract void UseTexture();
 }
 
 public class Test : TestBase
 {
     public Test () : base()
     {
     }
     
     public override void UseTexture()
     {
         // others
         
         texture.Resize(80, 80);
         
         // others code
     }
 }
 
 public class TestWindow : EditorWindow
 {
     private static TestBase base;
     
     [MenuItem("Tools/Test")]
     private static void Init()
     {
         // Initialization
     }
     
     public void OnGUI()
     {
         if (base == null)
         {
             base = new Test();
         }
         
         base.UseTexture();
     }
     
     public void OnDestroy()
     {
         // others
         
         Resources.UnloadUnusedAssets;
     }
 }

we can get a Editor Window. With the steps below, I can get MissingReferenceException, and the "texture" will be "null". 1. The "TestWindow" will be opened automatically at Play Mode; 2. Exit Play Mode, the "TestWindow" will be closed automatically; 3. Open the window from the Menu, then the Exception occurs.

Someone told me that set the hideFlags of Texture2D to be "DontSave", such as:

 public abstract class TestBase
 {
     ...
     public TestBase()
     {
         texture = new Texture2D(100, 100, TextureFormat.RGB24, false)
                       {
                           hideFlags = HideFlags.DontSave
                       };
     }
         ...
 }

The error is fixed. But I cannot actually get why.

Could someone explain it?

Thanks