NavMeshAdd.ForceUpdate attempts to force updates when you’re in editor, editing a prefab, not surprisingly not having a AstarPath.active. The following quick fix would remove this annoyance:
/// <summary>
/// Forces this navmesh add to update the navmesh.
///
/// This update is not instant, it is done the next time it is checked if it needs updating.
/// See: <see cref="NavmeshUpdates.updateInterval"/>
/// See: <see cref="NavmeshUpdates.ForceUpdate"/>
/// </summary>
public override void ForceUpdate () {
#if UNITY_EDITOR
if (!Application.isPlaying && AstarPath.active == null)
return;
#endif
AstarPath.active.navmeshUpdates.ForceUpdateAround(this);
}