Custom Shader GUI problem

I am having getting this error when I switch on/off the toggle in the custom shader GUI.

Error Code:

 InvalidOperationException: Operation is not valid due to the current state of the object
     System.Collections.Generic.Stack`1[System.Boolean].Pop ()
     UnityEditor.EditorGUI.EndChangeCheck () (at C:/buildslave/unity/build/Editor/Mono/EditorGUI.cs:234)
     UnityEditor.MaterialEditor.PropertiesGUI () (at C:/buildslave/unity/build/Editor/Mono/Inspector/MaterialEditor.cs:1224)
     UnityEditor.MaterialEditor.OnInspectorGUI () (at C:/buildslave/unity/build/Editor/Mono/Inspector/MaterialEditor.cs:235)
     UnityEditor.InspectorWindow.DrawEditor (UnityEditor.Editor editor, Int32 editorIndex, Boolean rebuildOptimizedGUIBlock, System.Boolean& showImportedObjectBarNext, UnityEngine.Rect& importedObjectBarRect) (at C:/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:1231)
     UnityEditor.DockArea:OnGUI()

My custom shadergui has this code:

 using System.Collections.Generic;
 using UnityEngine;
 using UnityEditor;
 using System.Linq;
 using System.Text.RegularExpressions;
 
 public abstract class EtlokShaderGUI : ShaderGUI
 {
     public class FeatureToggle
     {
         // The name the toggle will have in the inspector.
         public string InspectorName;
         // We will look for properties that contain this word, and hide them if we're not enabled.
         public string InspectorPropertyHideTag;
         // The keyword that the shader uses when this feature is enabled or disabled.
         public string ShaderKeywordEnabled;
         public string ShaderKeywordDisabled;
         // The current state of this feature.
         public bool Enabled;
 
         public FeatureToggle(string InspectorName, string InspectorPropertyHideTag, string ShaderKeywordEnabled, string ShaderKeywordDisabled)
         {
             this.InspectorName = InspectorName;
             this.InspectorPropertyHideTag = InspectorPropertyHideTag;
             this.ShaderKeywordEnabled = ShaderKeywordEnabled;
             this.ShaderKeywordDisabled = ShaderKeywordDisabled;
             this.Enabled = false;
         }
     }
 
     // A list of all the toggles that we have in this material editor.
     protected List<FeatureToggle> Toggles = new List<FeatureToggle>();
     // This function will be implemented in derived classes, and used to populate the list of toggles.
     protected abstract void CreateToggleList();
 
     public override void OnGUI(MaterialEditor materialEditor, MaterialProperty[] properties)
     {
         //base.OnGUI(materialEditor, properties);
 
         Material targetMat = materialEditor.target as Material;
         Shader shader = targetMat.shader;
 
         string[] oldKeyWords = targetMat.shaderKeywords;
 
         // Populate our list of toggles
         //Toggles.Clear();
         Toggles = new List<FeatureToggle>();
         CreateToggleList();
 
         // Update each toggle to enabled if it's enabled keyword is present. If it's enabled keyword is missing, we assume it's disabled.
         for (int i = 0; i < Toggles.Count; i++)
         {
             Toggles[i].Enabled = oldKeyWords.Contains(Toggles[i].ShaderKeywordEnabled);
         }
 
         EditorGUI.BeginChangeCheck();
 
         // Draw Non-toggleable values
         for (int i = 0; i < ShaderUtil.GetPropertyCount(shader); i++)
         {
             ShaderPropertyImpl(materialEditor, properties, i, null);
         }
         // Draw toggles, then their values.
         for (int s = 0; s < Toggles.Count; s++)
         {
             EditorGUILayout.Separator();
             Toggles[s].Enabled = EditorGUILayout.BeginToggleGroup(Toggles[s].InspectorName, Toggles[s].Enabled);
 
             if (Toggles[s].Enabled)
             {
                 for (int i = 0; i < ShaderUtil.GetPropertyCount(shader); i++)
                 {
                     ShaderPropertyImpl(materialEditor, properties, i, Toggles[s]);
                 }
             }
             EditorGUILayout.EndToggleGroup();
         }
 
         if (EditorGUI.EndChangeCheck())
             materialEditor.PropertiesChanged();
 
         // If changes have been made, then apply them.
         if (EditorGUI.EndChangeCheck())
         {
             // New list of key words.
             List<string> newKeyWords = new List<string>();
 
             // If true, add the enabled keyword (ending with _ON), if false, add the disabled keyword(ending with _OFF).
             for (int i = 0; i < Toggles.Count; i++)
             {
                 newKeyWords.Add(Toggles[i].Enabled ? Toggles[i].ShaderKeywordEnabled : Toggles[i].ShaderKeywordDisabled);
             }
 
             // Send the new list of keywords to the material, this will define what version of the shader to use.
             targetMat.shaderKeywords = newKeyWords.ToArray();
             EditorUtility.SetDirty(targetMat);
         }
     }
 
     private void ShaderPropertyImpl(MaterialEditor matEditor, MaterialProperty[] matpropset, int propertyIndex, FeatureToggle currentToggle)
     {
         Material targetMat = matEditor.target as Material;
         Shader shader = targetMat.shader;
 
         string propertyDescription = ShaderUtil.GetPropertyDescription(shader, propertyIndex);
         string propertyName = ShaderUtil.GetPropertyName(shader, propertyIndex);
 
         // If current toggle is null, we only want to show properties that aren't already "owned" by a toggle,
         // so if it is owned by another toggle, then return.
         if (currentToggle == null)
         {
             for (int i = 0; i < Toggles.Count; i++)
             {
                 if (Regex.IsMatch(propertyDescription, Toggles[i].InspectorPropertyHideTag, RegexOptions.IgnoreCase))
                 {
                     return;
                 }
             }
         }
         // Only draw if we the current property is owned by the current toggle.
         else if (!Regex.IsMatch(propertyDescription, currentToggle.InspectorPropertyHideTag, RegexOptions.IgnoreCase))
         {
             return;
         }
         // If we've gotten to this point, draw the shader property regulairly.
         MaterialProperty matprop = ShaderGUI.FindProperty(propertyName, matpropset);
         string matDisplayName = matprop.displayName;
         matEditor.ShaderProperty(matprop, matDisplayName);
     }
 }

With the custom inspector of :

 using System.Collections.Generic;
 using UnityEngine;
 using UnityEditor;
 public class EtlokInspector : EtlokShaderGUI
 {
     protected override void CreateToggleList()
     {
         Toggles.Add(new FeatureToggle("DiffuseMap Enabled", "diffuse", "DIFFUSEMAP_ON", "DIFFUSEMAP_OFF"));
         Toggles.Add(new FeatureToggle("AlphaMap Enabled", "alpha", "ALPHAMAP_ON", "ALPHAMAP_OFF"));
         Toggles.Add(new FeatureToggle("SpecularMap Enabled", "specular", "SPECULAR_ON", "SPECULAR_OFF"));
         Toggles.Add(new FeatureToggle("EnvironmentMap Enabled", "env", "ENVMAP_ON", "ENVMAP_OFF"));
         Toggles.Add(new FeatureToggle("BumpMap Enabled", "bump", "BUMPMAP_ON", "BUMPMAP_OFF"));
         Toggles.Add(new FeatureToggle("NormalMap Enabled", "normal", "NORMALMAP_ON", "NORMALMAP_OFF"));
         Toggles.Add(new FeatureToggle("EmissiveMap Enabled", "emissive", "EMISSIVEMAP_ON", "EMISSIVEMAP_OFF"));
     }
 }