C Sharp Error on play (editor hierarchy script)

Hi everyone, i intent to add Hierarchy Icons, because i have a very flat and large Hierarchy with hundreds of objects.
At the moment i use Tags for it, but that is only temporary till i find another way to do this.
I get no compiler complains until i go on Play (VR VIVE).

2 Things i must solve:

1.) i want to use the conditional statement in order to have this only in editor, but i don't know how to put the whole class into the statement, if i do so it still executes some code on play. ->

Code (csharp):

1. #if UNITY_EDITOR .... #endif

2.) on Play it tells me and error:

Code (csharp):

1.  
2. NullReferenceException: Object reference not set to an instance of an object
3. HierarchyLabels.HierarchyItemCB (Int32 instanceID, Rect selectionRect) (at Assets/_cutomScripts/MyHierarchyIcons.cs:86)
4.  

plus:

Code (csharp):

1.  
2. GUI Error: You are pushing more GUIClips than you are popping. Make sure they are balanced)
3. UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)
4.  

Of coarse if i can stop unity from executing this code on play this is obsolete...

any ideas or tipps?

it looks already fine:

Code (csharp):

1.  
2. using UnityEditor;
3. using UnityEngine;
4. using System.Collections.Generic;
5. //using UnityEngine.UI;
6.  
7.  [InitializeOnLoad]
8.  class HierarchyLabels
9.  {
10.     // m_hierarchyLabelTexture
11.     static Texture2D m_hierarchyLabelTexture;
12.     static List<int> m_markedhierarchyLabelObjects;
13.  
14.     // m_hierarchyLabelLight
15.     static Texture2D m_hierarchyLabelLightTexture;
16.     static List<int> m_markedLightLabelObjects;
17.  
18.     // m_hierarchyLabelInstance
19.     static Texture2D m_hierarchyLabelInstanceTexture;
20.     static List<int> m_markedInstanceLabelObjects;
21.  
22.     // this function muste be replaced with a editor input field to upload custom icons (future)
23.     static HierarchyLabels()
24.      {
25.         // Get Icon types from AssetDatabase Resource (folder)
26.         m_hierarchyLabelTexture = AssetDatabase.LoadAssetAtPath ("Assets/Resources/labels/hierarchy_label--gray.jpg", typeof(Texture2D)) as Texture2D;
27.         m_hierarchyLabelLightTexture = AssetDatabase.LoadAssetAtPath("Assets/Resources/labels/hierarchy_light_label.jpg", typeof(Texture2D)) as Texture2D;
28.         m_hierarchyLabelInstanceTexture = AssetDatabase.LoadAssetAtPath("Assets/Resources/labels/hierarchy_instance_label.jpg", typeof(Texture2D)) as Texture2D;
29.  
30.         EditorApplication.update += UpdateCB;
31.         EditorApplication.hierarchyWindowItemOnGUI += HierarchyItemCB;
32.      }
33.    
34.  
35.     // this function be updated to dynamicly get allicon types that have a tag accociated with
36.      static void UpdateCB ()
37.      {
38.          // collect all GameObject of type GameObject and laod em into array
39.          GameObject[] go = Object.FindObjectsOfType (typeof(GameObject)) as GameObject[];
40.  
41.  
42.         // hierarchy Labels
43.         m_markedhierarchyLabelObjects = new List<int> ();
44.  
45.         // hierarchy light Objects
46.         m_markedLightLabelObjects = new List<int>();
47.  
48.         // hierarchy Instanced (duplicated) Objects
49.         m_markedInstanceLabelObjects = new List<int>();
50.  
51.         foreach (GameObject g in go)
52.          {
53.             // mark all m_markedhierarchyLabelObjects
54.             if (g.CompareTag("HierarchyLabel"))
55.             {
56.                 m_markedhierarchyLabelObjects.Add(g.GetInstanceID());
57.              }
58.  
59.             // mark all m_markedLightLabelObjects
60.             if (g.CompareTag("LightGameObject"))
61.             {
62.                 m_markedLightLabelObjects.Add(g.GetInstanceID());
63.             }
64.  
65.             // mark all m_markedInstanceLabelObjects
66.             if (g.CompareTag("duplicatedGameObject"))
67.             {
68.                 m_markedInstanceLabelObjects.Add(g.GetInstanceID());
69.             }
70.         }
71.  
72.  
73.          
74.     }
75.  
76.      static void HierarchyItemCB (int instanceID, Rect selectionRect)
77.      {
78.         // place the icons to the right of the list:
79.  
80.             Rect r = new Rect (selectionRect);
81.             r.x = r.width - 40;
82.             r.width = 80;
83.  
84.        // Debug.Log(m_markedhierarchyLabelObjects.Contains(instanceID));
85.  
86.         if (m_markedhierarchyLabelObjects.Contains(instanceID))
87.          {
88.             GUI.Label(r, m_hierarchyLabelTexture);
89.             // Draw the texture if it has the label HierarchyLabel
90.         }
91.  
92.         if (m_markedLightLabelObjects.Contains(instanceID))
93.         {
94.             GUI.Label(r, m_hierarchyLabelLightTexture);
95.         }
96.  
97.         if (m_markedInstanceLabelObjects.Contains(instanceID))
98.         {
99.             GUI.Label(r, m_hierarchyLabelInstanceTexture);
100.         }
101.  
102.  
103.     }
104.  
105. }
106.  
107.  
108.