Property Drawer SerializedProperty Null inside Array

I'm new to this PropertyDrawer stuff. I have created PropertyDrawer for my class AudioData and it works fine. But when I create variable that is an array/list of AudioData and click the plus button, it throws a NullException to my property.FindPropertyRelative.

How do I fix this? Thanks.

 using UnityEngine;
 
 #if UNITY_EDITOR
 using UnityEditor;
 
 [CustomPropertyDrawer(typeof(AudioData), false)]
 public class AudioDataPropertyDrawer : PropertyDrawer
 {
     public override float GetPropertyHeight(SerializedProperty property, GUIContent label)
     {
         return property.isExpanded ? (EditorGUIUtility.singleLineHeight * 5) : EditorGUIUtility.singleLineHeight;
     }
 
     public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
     {
         EditorGUI.BeginProperty(position, label, property);
         if (property != null)
         {
             string foldOutName = string.IsNullOrEmpty(property.FindPropertyRelative("AudioName").stringValue) ? "Audio Data" : property.FindPropertyRelative("AudioName").stringValue;
             Rect foldOutRect = new Rect(position.x, position.y, position.width, 16);
             property.isExpanded = EditorGUI.Foldout(foldOutRect, property.isExpanded, foldOutName, true);
             EditorGUI.indentLevel++;
 
             if (property.isExpanded)
             {
                 Rect nameRect = new Rect(position.x, position.y + 18, position.width, 16);
                 EditorGUI.PropertyField(nameRect, property.FindPropertyRelative("AudioName"));
 
                 Rect clipRect = new Rect(position.x, position.y + 36, position.width, 16);
                 EditorGUI.PropertyField(clipRect, property.FindPropertyRelative("SoundClip"));
 
                 Rect isRandVolRect = new Rect(position.x, position.y + 54, position.width, 16);
                 EditorGUI.PropertyField(isRandVolRect, property.FindPropertyRelative("IsRandomizeVolume"));
 
                 Rect volumeRect = new Rect(position.x, position.y + 72, position.width, 16);
                 if (property.FindPropertyRelative("IsRandomizeVolume").boolValue)
                     EditorGUI.PropertyField(volumeRect, property.FindPropertyRelative("VolumeRange"));
                 else
                 {
                     Vector2 volX = property.FindPropertyRelative("VolumeRange").vector2Value;
                     volX.x = EditorGUI.Slider(volumeRect, "Volume", property.FindPropertyRelative("VolumeRange").vector2Value.x, 0, 1);
                     property.FindPropertyRelative("VolumeRange").vector2Value = volX;
                 }
             }
             EditorGUI.indentLevel--;
         }
         EditorGUI.EndProperty();
     }
 }
 #endif
 
 public enum SoundTypeEnum
 {
     SFX,
     BGM,
 }
 
 [System.Serializable]
 public class AudioData
 {
     public string AudioName = "";
     public AudioClip SoundClip = null;
     public bool IsRandomizeVolume = false;
     public Vector2 VolumeRange = Vector2.one;
     
     public virtual AudioClip GetSoundClip(object[] obj = null)
     {
         return SoundClip;
     }
     public virtual float GetVolume(object[] obj = null)
     {
         return IsRandomizeVolume ? Random.Range(VolumeRange.x, VolumeRange.y) : VolumeRange.x;
     }
 
     public AudioData()
     {
         AudioName = "";
         SoundClip = null;
         IsRandomizeVolume = false;
         VolumeRange = Vector2.one;
     }
 
     public AudioData(string name = "", AudioClip clip = null, bool isRandomizeVol = false, Vector2? vol = null)
     {
         AudioName = name;
         SoundClip = clip;
         IsRandomizeVolume = isRandomizeVol;
         VolumeRange = vol ?? Vector2.one;
     }
     public AudioData(AudioClip clip = null, bool isRandomizeVol = false, Vector2? vol = null)
     {
         AudioName = clip.name;
         SoundClip = clip;
         IsRandomizeVolume = isRandomizeVol;
         VolumeRange = vol ?? Vector2.one;
     }
 }

This is what happened in the Editor (Attached GIF):

The error:

 NullReferenceException: Object reference not set to an instance of an object
 AudioDataPropertyDrawer.OnGUI (UnityEngine.Rect position, UnityEditor.SerializedProperty property, UnityEngine.GUIContent label) (at Assets/_Project/Scripts/Managers/Audio Manager/AudioData.cs:19)
 UnityEditor.PropertyDrawer.OnGUISafe (UnityEngine.Rect position, UnityEditor.SerializedProperty property, UnityEngine.GUIContent label) (at <d0e4b8c1204147f3935b6e9c8b8565ad>:0)
 UnityEditor.PropertyHandler.OnGUI (UnityEngine.Rect position, UnityEditor.SerializedProperty property, UnityEngine.GUIContent label, System.Boolean includeChildren, UnityEngine.Rect visibleArea) (at <d0e4b8c1204147f3935b6e9c8b8565ad>:0)
 UnityEditor.PropertyHandler.OnGUI (UnityEngine.Rect position, UnityEditor.SerializedProperty property, UnityEngine.GUIContent label, System.Boolean includeChildren) (at <d0e4b8c1204147f3935b6e9c8b8565ad>:0)
 UnityEditorInternal.ReorderableList+Defaults.DrawElement (UnityEngine.Rect rect, UnityEditor.SerializedProperty element, System.Object listItem, System.Boolean selected, System.Boolean focused, System.Boolean draggable, System.Boolean editable) (at <d0e4b8c1204147f3935b6e9c8b8565ad>:0)
 UnityEditorInternal.ReorderableList.DoListElements (UnityEngine.Rect listRect, UnityEngine.Rect visibleRect) (at <d0e4b8c1204147f3935b6e9c8b8565ad>:0)
 UnityEditorInternal.ReorderableList.DoList (UnityEngine.Rect rect, UnityEngine.Rect visibleRect) (at <d0e4b8c1204147f3935b6e9c8b8565ad>:0)
 UnityEditorInternal.ReorderableListWrapper.Draw (UnityEngine.GUIContent label, UnityEngine.Rect r, UnityEngine.Rect visibleArea, System.Boolean includeChildren) (at <d0e4b8c1204147f3935b6e9c8b8565ad>:0)
 UnityEditor.PropertyHandler.OnGUI (UnityEngine.Rect position, UnityEditor.SerializedProperty property, UnityEngine.GUIContent label, System.Boolean includeChildren, UnityEngine.Rect visibleArea) (at <d0e4b8c1204147f3935b6e9c8b8565ad>:0)
 UnityEditor.PropertyHandler.OnGUI (UnityEngine.Rect position, UnityEditor.SerializedProperty property, UnityEngine.GUIContent label, System.Boolean includeChildren) (at <d0e4b8c1204147f3935b6e9c8b8565ad>:0)
 UnityEditor.PropertyHandler.OnGUILayout (UnityEditor.SerializedProperty property, UnityEngine.GUIContent label, System.Boolean includeChildren, UnityEngine.GUILayoutOption[] options) (at <d0e4b8c1204147f3935b6e9c8b8565ad>:0)
 UnityEditor.EditorGUILayout.PropertyField (UnityEditor.SerializedProperty property, UnityEngine.GUIContent label, System.Boolean includeChildren, UnityEngine.GUILayoutOption[] options) (at <d0e4b8c1204147f3935b6e9c8b8565ad>:0)
 UnityEditor.EditorGUILayout.PropertyField (UnityEditor.SerializedProperty property, System.Boolean includeChildren, UnityEngine.GUILayoutOption[] options) (at <d0e4b8c1204147f3935b6e9c8b8565ad>:0)
 UnityEditor.Editor.DoDrawDefaultInspector (UnityEditor.SerializedObject obj) (at <d0e4b8c1204147f3935b6e9c8b8565ad>:0)
 UnityEditor.Editor.DoDrawDefaultInspector () (at <d0e4b8c1204147f3935b6e9c8b8565ad>:0)
 UnityEditor.Editor.DrawDefaultInspector () (at <d0e4b8c1204147f3935b6e9c8b8565ad>:0)
 UnityEditor.Editor.OnInspectorGUI () (at <d0e4b8c1204147f3935b6e9c8b8565ad>:0)
 UnityEditor.UIElements.InspectorElement+<>c__DisplayClass59_0.<CreateIMGUIInspectorFromEditor>b__0 () (at <17cba49fae414354b79f447a7073fc5b>:0)
 UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr, Boolean&)