Object field not working in custom editor for scriptable object?

In the project I'm working on, we have a static Scriptable Object called Level Directory which holds a list of all level metadata (each of which are Scriptable Objects too). The idea was to make a Custom Editor for the Level Directory so that it'll display the list as a Reorderable List so that we could reorder our levels as we'd like. This is how it looks so far:

But it doesn't seem to be working. I can't assign any Level Data Scriptable Object to any of the Level IDs and then there's that Levels dropdown list that I'm not sure where it popped up from (it wasn't there a few hours ago). Am I missing something in my code? The custom editor script is as below:

 using UnityEditor;
 using UnityEditorInternal;
 using UnityEngine;
 
 [CustomEditor(typeof(LevelDirectory))]
 public class LevelDirectoryEditor : Editor
 {
     #region CONST STRINGS
     private const string levelNumberLabel = "Level No.";
     private const string levelIDLabel = "Level ID";
     private const string levelDirectoryLabel = "Level Directory";
     #endregion
 
     private ReorderableList reorderableList;
     public LevelDirectory levelDir { get { return (target as LevelDirectory); } }
 
     // CACHED VARIABLES
     private Object element = null;
 
     private void OnEnable()
     {
         reorderableList = new ReorderableList(levelDir.levels, typeof(LevelData), true, true, true, true);
         reorderableList.drawHeaderCallback = OnDrawHeader;
         reorderableList.drawElementCallback = OnDrawElement;
         reorderableList.onAddCallback = OnAdd;
         reorderableList.onChangedCallback = OnChanged;
     }
 
     /// <summary>
     /// Draws the header for the Level Directory reorderable list.
     /// </summary>
     /// <param name="rect"></param>
     private void OnDrawHeader(Rect rect)
     {
         Rect _rect = new Rect(rect.x, rect.y, 100, EditorGUIUtility.singleLineHeight);
 
         GUI.Label(_rect, levelNumberLabel);
         _rect.x += 100;
         GUI.Label(_rect, levelIDLabel);
     }
 
     /// <summary>
     /// A method to determine what to draw for each element on the list.
     /// </summary>
     /// <param name="rect"></param>
     /// <param name="index"></param>
     /// <param name="isActive"></param>
     /// <param name="isFocused"></param>
     private void OnDrawElement(Rect rect, int index, bool isActive, bool isFocused)
     {
         // get level data if not null
         element = levelDir.levels[index];
 
         rect.y += 2;
         Rect _rect = new Rect(rect.x, rect.y, 100, EditorGUIUtility.singleLineHeight);
 
         // display index
         EditorGUI.LabelField(_rect, (index + 1).ToString());
         _rect.x += 100;
         _rect.width = 150;
         // display level data asset
         element = EditorGUI.ObjectField(_rect, element, typeof(LevelData), false);
     }
 
     /// <summary>
     /// Defines how a new Level Data asset should be added to the list.
     /// </summary>
     /// <param name="list"></param>
     private void OnAdd(ReorderableList list)
     {
         levelDir.levels.Add(null);
     }
 
     private void OnChanged(ReorderableList list)
     {
         EditorUtility.SetDirty(target);
     }
 
     public override void OnInspectorGUI()
     {
         EditorGUILayout.LabelField(levelDirectoryLabel);
 
         EditorGUILayout.Space();
 
         reorderableList.DoLayoutList();
     }
 }