"You can only call GUI functions from inside OnGUI." Each time I try to open a window after a recompilation when the window was opened?

This is really annoying - Say I have a EditorWindow opened - and then make something that triggers a recompilation - I start to get "You can only call GUI functions from inside OnGUI." about my window (also for each time I try to open it from this moment on). The only way to fix this is to close the window and then trigger a recompilation again.

Why does this happen? -__-

Thanks!

EDIT: Code

 using UnityEngine;
 using System.Collections.Generic;
 using UnityEditor;
 using System.Linq;
 using System.Text.RegularExpressions;

 public class SelectSceneWindow : EditorWindow
 {
     /* a bunch of GUIStyles... */

     private SceneTransition transition;
     private Object[] scenes;
     private Object[] filteredResult;

     public static void Show(SceneTransition transition)
     {
         var window = CreateInstance<SelectSceneWindow>();
         window.Init(transition);
         window.ShowUtility();
     }

     public void Init(SceneTransition transition)
     {
         this.transition = transition;
     }

     Vector2 scrollPosition;
     string search = "";
     private const float INDENT_WIDTH = 20f;

     void OnGUI()
     {
         GUIHelper.HorizontalBlock(() =>
         {
             GUILayout.Label("Scenes", ScenesLabel);
             search = EditorGUILayout.TextField("", search);
             filteredResult = scenes.Where(s => Regex.IsMatch(s.name, search, RegexOptions.IgnoreCase)).ToArray();
             if (GUILayout.Button(new GUIContent("↶", "Reload scenes"), RefreshButton, GUILayout.Width(20), GUILayout.Height(20)))
                 ReloadScenes();
         });

         scrollPosition = IUnifiedGUIHelper.ScrollViewBlock(scrollPosition, false, false, () =>
         {
             foreach (var scene in filteredResult) {
                 bool selected = scene.name == transition.scene;
                 var nextStyle = GetNextStyle();
                 IUnifiedGUIHelper.HorizontalBlock(selected ? SelectedStyle : nextStyle, () =>
                 {
                     GUILayout.Space(INDENT_WIDTH);
                     var spaceRect = GUILayoutUtility.GetLastRect();
                     GUILayout.Label(scene.name, selected ? SelectedLabel : UnselectedLabel);
                     var labelRect = GUILayoutUtility.GetLastRect();
                     var buttonRect = IUnifiedGUIHelper.CombineRects(spaceRect, labelRect);
                     if (!selected) {
                         EditorGUIUtility.AddCursorRect(buttonRect, MouseCursor.Link);
                         if (GUI.Button(buttonRect, "", GUIStyle.none)) {
                             transition.scene = scene.name;
                         }
                     }
                 });
             }
         });
     }
     void ReloadScenes()
     {
         scenes = Utils.GetAssets<Object>("Assets", "*.unity", SearchOption.AllDirectories);
         filteredResult = scenes;
     }

     void OnEnable()
     {
         ReloadScenes();
     }
 }

What it looks like