Error CS0122: inaccessable due to protection level

I've had no problem with this script until yesterday, I'm fairly new to coding, but I've gone through this several times and can't find the problem? The full error is

Assets/Editor/ImageEffects/CreaseShadingEditor.cs(8,23): error CS0122: `CreaseShading' is inaccessible due to its protection level

But I can't find an issue on line 8 or anywhere else? Am I just being dim?

 using System;
 using UnityEditor;
 using UnityEditor.AnimatedValues;
 using UnityEngine;
 
 namespace UnityStandardAssets.ImageEffects
 {
     [CustomEditor(typeof(CreaseShading))]
     class CreaseShadingEditor : Editor {
         SerializedObject serObj;
 
         SerializedProperty m_intensity;
         SerializedProperty m_softness;
         SerializedProperty m_spread;
 
         AnimBool m_showSoftnessWarning = new AnimBool();
         AnimBool m_showSpreadWarning = new AnimBool();
 
         bool softnessWarningValue { get { return m_softness.intValue > 4; } }
         bool spreadWarningValue { get { return m_spread.floatValue > 4; } }
 
         void OnEnable () {
             serObj = new SerializedObject (target);
 
             m_intensity = serObj.FindProperty("intensity");
             m_softness = serObj.FindProperty("softness");
             m_spread = serObj.FindProperty("spread");
 
             m_showSoftnessWarning.valueChanged.AddListener(Repaint);
             m_showSpreadWarning.valueChanged.AddListener(Repaint);
 
             m_showSoftnessWarning.value = softnessWarningValue;
             m_showSpreadWarning.value = spreadWarningValue;
         }
 
         public override void OnInspectorGUI () {
             serObj.Update ();
 
             EditorGUILayout.Slider(m_intensity, -5.0f, 5.0f, new GUIContent("Intensity"));
 
             EditorGUILayout.IntSlider(m_softness, 0, 15, new GUIContent("Softness"));
             m_showSoftnessWarning.target = softnessWarningValue;
             if (EditorGUILayout.BeginFadeGroup(m_showSoftnessWarning.faded))
             {
                 EditorGUILayout.HelpBox("High Softness value might reduce performance.", MessageType.Warning, false);
             }
             EditorGUILayout.EndFadeGroup();
 
             EditorGUILayout.Slider(m_spread, 0.0f, 50.0f, new GUIContent("Spread"));
             m_showSpreadWarning.target = spreadWarningValue;
             if (EditorGUILayout.BeginFadeGroup(m_showSpreadWarning.faded))
             {
                 EditorGUILayout.HelpBox("High Spread value might introduce visual artifacts.", MessageType.Warning, false);
             }
             EditorGUILayout.EndFadeGroup();
 
             serObj.ApplyModifiedProperties ();
         }
     }
 }