diff --git a/Editor/ClusterViewer.cs b/Editor/ClusterViewer.cs
index 11013d3..54df2e8 100644
--- a/Editor/ClusterViewer.cs
+++ b/Editor/ClusterViewer.cs
@@ -99,7 +99,9 @@ namespace NanoBrain {
 
             public void SetGraph(GameObject gameObject) {
                 this.gameObject = gameObject;
-
+                if (this.currentCluster == null)
+                    return;
+                    
                 if (Application.isPlaying == false)
                     this.serializedBrain = new SerializedObject(this.currentCluster.prefab);
                 this.selectedOutput = this.currentCluster.outputs[0];
@@ -121,7 +123,7 @@ namespace NanoBrain {
             }
 
             public void OnIMGUI() {
-                if (Application.isPlaying == false)
+                if (Application.isPlaying == false && serializedBrain != null)
                     serializedBrain.Update();
 
                 Handles.BeginGUI();