NullReferenceException Error when trying terrain hlod

[youngwoocho02]

what is this error and how to fix it?

NullReferenceException: Object reference not set to an instance of an object
Unity.HLODSystem.SpaceManager.QuadTreeSpaceSplitter.CreateTargetInfoList (System.Collections.Generic.List1[T] gameObjects, UnityEngine.Transform transform) (at C:/Unity Workspace/HLOD/HLODSystem/com.unity.hlod/Editor/SpaceManager/QuadTreeSpaceSplitter.cs:214) Unity.HLODSystem.SpaceManager.QuadTreeSpaceSplitter.CreateSpaceTree (UnityEngine.Bounds initBounds, System.Single chunkSize, UnityEngine.Transform transform, System.Collections.Generic.List1[T] targetObjects, System.Action`1[T] onProgress) (at C:/Unity Workspace/HLOD/HLODSystem/com.unity.hlod/Editor/SpaceManager/QuadTreeSpaceSplitter.cs:102)
Unity.HLODSystem.TerrainHLODCreator+d__35.MoveNext () (at C:/Unity Workspace/HLOD/HLODSystem/com.unity.hlod/Editor/TerrainHLODCreator.cs:1100)
Unity.HLODSystem.Utils.CustomCoroutine.MoveNext () (at C:/Unity Workspace/HLOD/HLODSystem/com.unity.hlod/Runtime/Utils/CoroutineRunner.cs:47)
Unity.HLODSystem.Utils.CoroutineRunner.EditorUpdate () (at C:/Unity Workspace/HLOD/HLODSystem/com.unity.hlod/Runtime/Utils/CoroutineRunner.cs:134)
UnityEditor.EditorApplication.Internal_CallUpdateFunctions () (at <6a5b55f2e18b419e9faedac06ac6af94>:0)