Drawing a scene in the inspector GUI

I'm writing an editor script which draws a preview scene in the inspector GUI. Basically, I load a prefab with a camera component into a preview scene and save a reference to the camera. Then I try to draw the scene onto a texture using that camera. My current approach doesn't seem to be working, or maybe there's something wrong in my code. I'd appreciate any help.

Below is some of my code:

 [CustomEditor(typeof(NPCSpawnConfig))]
 public class NPCSpawnEditor : Editor
 {
     enum SupportedAspects
     {
         Aspect4by3 = 1,
         Aspect5by4 = 2,
         Aspect16by10 = 3,
         Aspect16by9 = 4
     };
 
     Camera _cam = null;
     RenderTexture _rt;
     Texture2D _tex2d;
     Scene _scene;
 
     // preview variables
     SupportedAspects _aspectChoiceIdx = SupportedAspects.Aspect16by10;
     float _curAspect;
     // world space (orthographicSize)
     float _worldScreenHeight = 5;
     int _renderTextureHeight = 1080;
 
     float ToFloat(SupportedAspects aspects)
     {
         switch(aspects)
         {
             case SupportedAspects.Aspect16by10:
                 return 16 / 10f;
             case SupportedAspects.Aspect16by9:
                 return 16 / 9f;
             case SupportedAspects.Aspect4by3:
                 return 4 / 3f;
             case SupportedAspects.Aspect5by4:
                 return 5 / 4f;
             default:
                 throw new ArgumentException();
         }
     }
 
     void DrawRefScene()
     {
         _rt = new RenderTexture(Mathf.RoundToInt(_curAspect * _renderTextureHeight), _renderTextureHeight, 16);
         _cam.targetTexture = _rt;
         _cam.Render();
         _tex2d = new Texture2D(_rt.width, _rt.height, TextureFormat.RGBA32, false);
         _tex2d.Apply(false);
         Graphics.CopyTexture(_rt, _tex2d);
     }
 
     Vector2 GetGUIPreviewSize()
     {
         Vector2 camSizeWorld = new Vector2(_worldScreenHeight * _curAspect, _worldScreenHeight);
         float scaleFactor = EditorGUIUtility.currentViewWidth / camSizeWorld.x;
         return new Vector2(EditorGUIUtility.currentViewWidth, scaleFactor * camSizeWorld.y);
     }
 
     #region Init
     void OnEnable()
     {
         void OpenSceneDelay()
         {
             EditorApplication.delayCall -= OpenSceneDelay;
             DrawRefScene();
         }
 
         _aspectChoiceIdx = SupportedAspects.Aspect16by10;
 
         _scene = EditorSceneManager.NewPreviewScene();
 
         PrefabUtility.LoadPrefabContentsIntoPreviewScene("Assets/Prefabs/Demo/DemoBkg.prefab", _scene);
         _cam = _scene.GetRootGameObjects()[0].GetComponentInChildren<Camera>();
 
         _curAspect = ToFloat(_aspectChoiceIdx);
         _cam.aspect = _curAspect;
         _cam.orthographicSize = _worldScreenHeight;
 
         EditorApplication.delayCall += OpenSceneDelay;
     }
 
     void OnDisable()
     {
         EditorSceneManager.ClosePreviewScene(_scene);
     }
     #endregion
 
     void OnCamSettingChange()
     {
         _curAspect = ToFloat(_aspectChoiceIdx);
         _cam.aspect = _curAspect;
         _cam.orthographicSize = _worldScreenHeight;
         DrawRefScene();
     }
 
     // GUI states
     class GUIControlStates
     {
         public bool foldout = false;
     };
     GUIControlStates _guiStates = new GUIControlStates();
     public override void OnInspectorGUI()
     {
         // draw serializedObject fields
         // ....
 
         
         // display options
         using (var scope = new EditorGUI.ChangeCheckScope())
         {
             _aspectChoiceIdx = (SupportedAspects)EditorGUILayout.EnumPopup("label", (Enum)_aspectChoiceIdx);
             if (scope.changed)
             {
                 OnCamSettingChange();
             }
         }
         _guiStates.foldout = EditorGUILayout.Foldout(_guiStates.foldout, "label", true);
         if(_guiStates.foldout)
         {
             using (var scope = new EditorGUI.ChangeCheckScope())
             {
                 _worldScreenHeight = EditorGUILayout.FloatField("label", _worldScreenHeight);
                 _renderTextureHeight = EditorGUILayout.IntField("label", _renderTextureHeight);
 
                 if (scope.changed)
                 {
                     OnCamSettingChange();
                 }
             }
         }
 
         if (_tex2d != null)
         {
             Vector2 sz = GetGUIPreviewSize();
             Rect r = EditorGUILayout.GetControlRect(false,
                 GUILayout.Height(sz.y),
                 GUILayout.ExpandHeight(false));
             EditorGUI.DrawPreviewTexture(r, _tex2d);
         }
     }
 }
 

Here is the result: (only clear color is displayed, but the prefab contains a lot of sprites which should be drawn. The camera is also correctly positioned relative to the sprites.)