Why is my SerializedProperty.objectReferenceValue Not Accepting my Object?

In the following code...

     [CustomEditor(typeof(Unit))]
     public class UnitEditor : Editor
     {
         private Editor behaviourEditor;
 
         public override void OnInspectorGUI()
         {
             base.serializedObject.Update();
 
             SerializedProperty behaviourProperty = base.serializedObject.FindProperty("behaviour");
             UnitBehaviour behaviour = behaviourProperty.objectReferenceValue as UnitBehaviour;
             MonoScript behaviourScript = behaviour != null ? MonoScript.FromScriptableObject(behaviour) : null;
 
             EditorGUI.BeginChangeCheck();
             {
                 behaviourScript = EditorGUILayout.ObjectField("Behaviour", behaviourScript, typeof(MonoScript)) as MonoScript;
             }
             if (EditorGUI.EndChangeCheck())
             {
                 if (behaviour != null)
                 {
                     DestroyImmediate(behaviour);
                     behaviour = null;
                 }
                 if (behaviourScript != null && behaviourScript.GetClass().IsSubclassOf(typeof(UnitBehaviour)))
                 {
                     behaviour = ScriptableObject.CreateInstance(behaviourScript.GetClass()) as UnitBehaviour;
                 }
             }
 
             if (behaviour != null)
             {
                 CreateCachedEditor(behaviour, null, ref this.behaviourEditor);
                 EditorGUILayout.Space();
                 this.behaviourEditor.OnInspectorGUI();
             }
 
             Debug.Log(behaviour + " : Before");// Not Null
             behaviourProperty.objectReferenceValue = behaviour;// <----- This isn't working aperently
             Debug.Log(behaviourProperty.objectReferenceValue + " : After");// Still Null
 
             base.serializedObject.ApplyModifiedProperties();
         }
     }

I have a ScriptableObject of Unit and an abstract ScriptableObject of UnitBehaviour. My goal is to have only Unit stored as assets and pass in a MonoScript to declare its UnitBehaviour and embed it's inspector within the Unit's inspector. I used debug messages to figure out the issue and everything seems to be working fine except at the end when I go to apply the new UnitBehaviour to the SerializedProperty. As you can see with my comments set its value and it still shows as null. As a result applying the property wont really do much and on the next GUI update it will think the MonoScript is null and erase everything.