Re-apply fix for 1262826 that got lost in translation during the move to the Graphics repository (#3648)

Author: Chman

com.unity.postprocessing/CHANGELOG.md

@@ -10,6 +10,9 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - Support for the PlayStation 5 platform has been added.
 - Support for the XboxSeries platform has been added.
 
+### Fixed
+- Fix for issue thrown upon unloading a scene from an AssetBundle (case 1262826)
+
 ## [3.0.3] - 2021-02-19
 
 - bump version to 3.0.3 to not conflict with unrelease 3.0.2

com.unity.postprocessing/PostProcessing/Runtime/PostProcessLayer.cs

@@ -856,7 +856,10 @@ void SetupContext(PostProcessRenderContext context)
         {
             // Juggling required when a scene with post processing is loaded from an asset bundle
             // See #1148230
-            if (m_OldResources != m_Resources)
+            // Additional !RuntimeUtilities.isValidResources() to fix #1262826
+            // The static member s_Resources is unset by addressable. The code is ill formed as it
+            // is not made to handle multiple scene.
+            if (m_OldResources != m_Resources || !RuntimeUtilities.isValidResources())
             {
                 RuntimeUtilities.UpdateResources(m_Resources);
                 m_OldResources = m_Resources;

com.unity.postprocessing/PostProcessing/Runtime/Utils/RuntimeUtilities.cs

@@ -381,6 +381,11 @@ public static PropertySheet copyFromTexArraySheet
             }
         }
 
+        internal static bool isValidResources()
+        {
+            return s_Resources != null;
+        }
+
         internal static void UpdateResources(PostProcessResources resources)
         {
             Destroy(s_CopyMaterial);