Property 'UnityEditor.Editor.preview' is read only.

hi im trieing to get these compile errors away but this one i dont understand.

 Property 'UnityEditor.Editor.preview' is read only.

this is the full code:

 #pragma strict
 
 @CustomEditor (CameraMotionBlur)
 class CameraMotionBlurEditor extends Editor 
 {    
     var serObj : SerializedObject;    
         
   var filterType : SerializedProperty;
   var preview_ : SerializedProperty;
   var previewScale : SerializedProperty;
   var movementScale : SerializedProperty;
   var jitter : SerializedProperty;
   var rotationScale : SerializedProperty;
   var maxVelocity : SerializedProperty;
   var minVelocity : SerializedProperty;
   var maxNumSamples : SerializedProperty;
   var velocityScale : SerializedProperty;
   var velocityDownsample : SerializedProperty;
   var noiseTexture : SerializedProperty;
   var showVelocity : SerializedProperty;
   var showVelocityScale : SerializedProperty;
   var excludeLayers : SerializedProperty;
   //var dynamicLayers : SerializedProperty;
 
     function OnEnable () {
         serObj = new SerializedObject (target);
         
     filterType = serObj.FindProperty ("filterType");
 
     preview = serObj.FindProperty ("preview_");
     previewScale = serObj.FindProperty ("previewScale");
 
     movementScale = serObj.FindProperty ("movementScale");
     rotationScale = serObj.FindProperty ("rotationScale");
 
     maxVelocity = serObj.FindProperty ("maxVelocity");
     minVelocity = serObj.FindProperty ("minVelocity");
 
     maxNumSamples = serObj.FindProperty ("maxNumSamples");
     jitter = serObj.FindProperty ("jitter");
 
     excludeLayers = serObj.FindProperty ("excludeLayers");
     //dynamicLayers = serObj.FindProperty ("dynamicLayers");
 
     velocityScale = serObj.FindProperty ("velocityScale");
     velocityDownsample = serObj.FindProperty ("velocityDownsample");
 
     noiseTexture = serObj.FindProperty ("noiseTexture");
     } 
             
   function OnInspectorGUI () {         
     serObj.Update ();
                     
     EditorGUILayout.LabelField("Simulates camera based motion blur", EditorStyles.miniLabel);
 
     EditorGUILayout.PropertyField (filterType, new GUIContent("Technique"));      
     if (filterType.enumValueIndex == 3 && !(target as CameraMotionBlur).Dx11Support()) {
       EditorGUILayout.HelpBox("DX11 mode not supported (need shader model 5)", MessageType.Info);      
     }          
     EditorGUILayout.PropertyField (velocityScale, new GUIContent(" Velocity Scale"));   
     if(filterType.enumValueIndex >= 2) {
       EditorGUILayout.LabelField(" Tile size used during reconstruction filter:", EditorStyles.miniLabel);      
       EditorGUILayout.PropertyField (maxVelocity, new GUIContent("  Velocity Max"));  
     }
     else
       EditorGUILayout.PropertyField (maxVelocity, new GUIContent(" Velocity Max"));       
     EditorGUILayout.PropertyField (minVelocity, new GUIContent(" Velocity Min"));   
 
     EditorGUILayout.Separator ();
 
     EditorGUILayout.LabelField("Technique Specific");
 
     if(filterType.enumValueIndex == 0) {
       // portal style motion blur
       EditorGUILayout.PropertyField (rotationScale, new GUIContent(" Camera Rotation"));
       EditorGUILayout.PropertyField (movementScale, new GUIContent(" Camera Movement"));
     }
     else {
       // "plausible" blur or cheap, local blur
       EditorGUILayout.PropertyField (excludeLayers, new GUIContent(" Exclude Layers"));
       EditorGUILayout.PropertyField (velocityDownsample, new GUIContent(" Velocity Downsample"));
       velocityDownsample.intValue = velocityDownsample.intValue < 1 ? 1 : velocityDownsample.intValue;
       if(filterType.enumValueIndex >= 2) { // only display jitter for reconstruction
         EditorGUILayout.PropertyField (noiseTexture, new GUIContent(" Sample Jitter"));
         EditorGUILayout.PropertyField (jitter, new GUIContent("  Jitter Strength"));
       }
     }
 
     EditorGUILayout.Separator ();
 
     EditorGUILayout.PropertyField (preview, new GUIContent("Preview"));
     if (preview_.boolValue)
       EditorGUILayout.PropertyField (previewScale, new GUIContent(""));    
             
     serObj.ApplyModifiedProperties();
     }
 }
 

its in this line: preview = serObj.FindProperty ("preview");