[UUM-100205] Fix exception when changing SG variant limit under certain conditions

Author: dhsavell

Packages/com.unity.shadergraph/Editor/Data/Graphs/GraphData.cs

@@ -2875,6 +2875,8 @@ public void OnEnable()
                 node.OnEnable();
             }
 
+            // OnEnable may be called multiple times. Ensure the callback only exists once.
+            ShaderGraphPreferences.onVariantLimitChanged -= OnKeywordChanged;
             ShaderGraphPreferences.onVariantLimitChanged += OnKeywordChanged;
         }