Fix Editor Issue for LiftGammeGain and ShadowsMidtonesHighlights (#6813)

* Initial commit

* Changelog

Co-authored-by: sebastienlagarde <sebastien@unity3d.com>

com.unity.render-pipelines.high-definition/CHANGELOG.md

@@ -106,6 +106,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - Fixed debug data for probes to not longer be cleared every time a cell is added/removed. This helps performance with streaming.
 - Fixed APV loading data outside of the relevant area containing probes.
 - Fixed the roughness value used for screen space reflections and ray traced reflections to match environment lighting (case 1390916).
+- Fixed editor issue with the LiftGammaGain and ShadowsMidtonesHighlights volume components.
 
 ## [14.0.0] - 2021-11-17
 

com.unity.render-pipelines.high-definition/Editor/PostProcessing/TrackballUIDrawer.cs

@@ -11,17 +11,12 @@ sealed class TrackballUIDrawer
         static GUIStyle s_WheelThumb;
         static Vector2 s_WheelThumbSize;
         static Material s_Material;
+        static bool s_MaterialConfigured;
 
         Func<Vector4, Vector3> m_ComputeFunc;
         bool m_ResetState;
         Vector2 m_CursorPos;
 
-        static TrackballUIDrawer()
-        {
-            if (s_Material == null)
-                s_Material = new Material(Shader.Find("Hidden/HD PostProcessing/Editor/Trackball")) { hideFlags = HideFlags.HideAndDontSave };
-        }
-
         public void OnGUI(SerializedProperty property, SerializedProperty overrideState, GUIContent title, Func<Vector4, Vector3> computeFunc)
         {
             if (property.propertyType != SerializedPropertyType.Vector4)
@@ -30,6 +25,13 @@ public void OnGUI(SerializedProperty property, SerializedProperty overrideState,
                 return;
             }
 
+            if (!s_MaterialConfigured)
+            {
+                // Initialization of materials with Shader.Find from static constructors is not allowed.
+                s_Material = new Material(Shader.Find("Hidden/HD PostProcessing/Editor/Trackball")) { hideFlags = HideFlags.HideAndDontSave };
+                s_MaterialConfigured = true;
+            }
+
             m_ComputeFunc = computeFunc;
             var value = property.vector4Value;