SerializedObject.FindProperty returning null

Firstly I am missing my inheritFrom field when using the normal Inspector (not anymore when I inherit from UnityEngine.Object, then there's just 1 field):

 [DataContract]
 [System.Serializable]
 public class BotPersonality : UnityEngine.Object
 {

 [DataMember]
 [JsonProperty]
 [SerializeField]
 private BotPersonality _inheritFrom;
 
 [DataMember]
 [JsonProperty]
 [SerializeField]
 private string[] _possibleNames = {"test1", "test2"};

I want to write a custom inspector for inheriting / copying fields from another object with _inheritFrom.

However I am getting NullReferenceExceptions ( SerializedObject.FindProperty returns null) for all my fields.

it says it can't find the skill property, but it is clearly defined and it works in the first image.:

 [DataMember]
 [SerializeField]
 public int skill = 1;

The skill variable is serializable.

Below is my PropertyDrawer class

     [CustomPropertyDrawer(typeof(BotPersonality))]
     public class BotPersonalityDrawer : PropertyDrawer
     {
         private readonly Type _type = typeof(BotPersonality);

 public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
 {
     BotPersonality botPersonality =
         Utils.GetActualObjectForSerializedProperty<BotPersonality>(this.fieldInfo, property);    
     this.ShowPersonality(botPersonality, property);
 }

     private void ShowPersonality(BotPersonality bot, SerializedProperty property)
     {
         int prevIndentLevel = EditorGUI.indentLevel;
          foreach (FieldInfo fieldType in this._type.GetFields()
                         .Where(x => x.IsDefined(typeof(DataMemberAttribute), false)))
         {
         ...
         var serializedProperty = property.serializedObject.FindProperty(fieldType.Name);
             EditorGUILayout.PropertyField(serializedProperty);
         }
 }
 }

The code for GetActualObjectForSerializedProperty can be found here: http://sketchyventures.com/2015/08/07/unity-tip-getting-the-actual-object-from-a-custom-property-drawer/ (I changed it so it works with lists too ), but this an object that is not null.

Help would be much appreciated. I can't see what I'm doing wrong.

EDIT: To clarify, I'm not looking for help with the inheritance / copy an object.

EDIT2: I've created it as small as I can and it still gives the same NullRef Exception:

 [DataContract]
 [System.Serializable]
 public class BotPersonalitySmall : MonoBehaviour
 {
     [DataMember]
     [SerializeField]
     public int skill = 1;
     [DataMember]
     [SerializeField]
     private BotPersonalitySmall _inheritFrom;
     [DataMember]
     [Overridable]
     public BotPersonality.WaypointSteeringInfo waypointSteering;
 }

The class I'm trying to serialize

The PropertyDrawer:

 [CustomPropertyDrawer(typeof(BotPersonalitySmall))]
 public class BotPersonalitySmallDrawer : PropertyDrawer
 {
     public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
     {
         // Using BeginProperty / EndProperty on the parent property means that
         // prefab override logic works on the entire property.
         EditorGUI.BeginProperty(position, label, property);
         var serializedProperty = property.serializedObject.FindProperty("skill");
         EditorGUILayout.PropertyField(serializedProperty);
         EditorGUI.EndProperty();
     }
 }

It still can't find the property skill. I've changed the inheritance on BotPersonality to UnityEngine.Object or keep it empty, but that didn't help.