Question Why the timeline asset loaded at OnGraphStart() is been destroyed?

I want jump to the second tineline when the first is end. I load the second timeline asset at
OnGraphStart() method of the lastest timeline clip in the first timeline. But when the clip start,unity show "MissingReferenceException: The object of type 'TimelineAsset' has been destroyed but you are still trying to access it." Later, I test the Sprite-Type variable and find sprite can be accessed. Someone know about this?
code:

Code (CSharp):

1. public class TestBehaviour : PlayableBehaviour
2.  
3. {
4.     private TimelineAsset testAsset;
5.     private Sprite testImage;
6.  
7.     // Called when the owning graph starts playing
8.     public override void OnGraphStart(Playable playable)
9.     {
10.         Debug.Log($"graph start");
11.         Test();
12.     }
13.  
14.     async void Test()
15.     {
16.         testAsset = (TimelineAsset)await GameEntry.Resource.LoadAssetAsync<PlayableAsset>(assetPath);
17.         testImage = await GameEntry.Resource.LoadAssetAsync<Sprite>(spritePath);
18.         Debug.Log($"TestAsset1:{testAsset.name}");
19.         Debug.Log($"TestSprite1:{testImage.name}");
20.     }
21.  
22.     // Called when the state of the playable is set to Play
23.     public override void OnBehaviourPlay(Playable playable, FrameData info)
24.     {
25.         Debug.Log($"behaviour start");
26.         Debug.Log($"TestAsset2:{testImage.name}");
27.         Debug.Log($"TestSprite2:{testAsset.name}");
28.      
29.     }
30. }

Result: