if I select the cinematic track and export selected clips , it still has the same error
fbxexport package :

IExporteData.cs
private static GameObject GetGameObjectBoundToTimelineClip(TimelineClip timelineClip)
{
object parentTrack = timelineClip.GetParentTrack();
AnimationTrack animTrack = parentTrack as AnimationTrack;

Object animationTrackObject = UnityEditor.Timeline.TimelineEditor.inspectedDirector.GetGenericBinding(animTrack);

the animTrack is Null , it seems that virtual camera brain track can not be treated as an animtrack so that the object is null

I get the same error too.

1. Select any object in a scene.
2. Window > Sequencing > Timeline > Click "Create" button
3. In Timeline window, check recording button, move time line and move object > animation is created.
4. In Timeline window, select animator > menu > Convert to Clip Track
5. Select timeline clip named "Recorded"
6. On Unity's top menu, select Game Object > Export Selected Timeline Clip

will results the following error:

NullReferenceException: Object reference not set to an instance of an object
FbxExporters.Editor.ModelExporter.ExportSingleEditorClip (UnityEngine.Object editorClipSelected) (at <ed6d9bae7a3e4e4ea6fa1b1d658e00cb>:0)
FbxExporters.Editor.ModelExporter.OnClipContextClick (UnityEditor.MenuCommand command) (at <ed6d9bae7a3e4e4ea6fa1b1d658e00cb>:0)

I would get this error too. My solution (i only needed to export geo, no timeline) was to make a new prefab of of the node i wanted to export. then "unpack prefab completely" and then i could export it. having fbx files in the hierarchy i think is the problem. unpacking them solved it, i think.