Custom Timeline Behaviour - GameObject can only be selected from Project Folder - not from Scene.

I created a custom Behaviour and I added a GameObject Field to it.

 using UnityEngine;
 using UnityEngine.Playables;
 public class LocalizedDialogTimeline : PlayableAsset
 {
     public string dialogId;
     public GameObject gameObject;
 
 
     public override Playable CreatePlayable(PlayableGraph graph, GameObject owner)
     {
         var scriptPlayable = ScriptPlayable<LocalizedDialogTimelineBehaviour>.Create(graph);
         var localizedDialogTimelineBehaviour = scriptPlayable.GetBehaviour();
         localizedDialogTimelineBehaviour.dialogId = dialogId;
         localizedDialogTimelineBehaviour.gameObject = gameObject;
         return scriptPlayable;
     }    
 }

 using TMPro;
 using UnityEngine;
 using UnityEngine.Playables;
 public class LocalizedDialogTimelineBehaviour : PlayableBehaviour
 {
     public string dialogId;
     public GameObject gameObject;
 }

So you should be able to enter an Id (which works like charm) and you should be able to select a GameObject. I cannot select GameObjects from the Scene. I can only select assets from my Asset folder.

I'm kinda confused why this happens. How can I select a scene GameObject in a custom timeline behaviour?