[HDRP] fix nullref when chaging RP from HDRP to URP

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

@@ -27,6 +27,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - Fixed light gizmo showing shadow near plane when shadows are disabled.
 - Fixed path tracing alpha channel support (case 1304187).
 - Fixed shadow matte not working with ambient occlusion when MSAA is enabled
+- Fixed exception when changing the current render pipeline to from HDRP to universal (case 1306291).
 
 ### Changed
 - Change the source value for the ray tracing frame index iterator from m_FrameCount to the camera frame count (case 1301356).

com.unity.render-pipelines.high-definition/Runtime/Lighting/VolumetricLighting/DensityVolumeManager.cs

@@ -124,8 +124,11 @@ public List<DensityVolume> PrepareDensityVolumeData(CommandBuffer cmd, HDCamera
         internal void ReleaseAtlas()
         {
             // Release the atlas so next time the manager is used, it is reallocated with new HDRP settings.
-            volumeAtlas.Release();
-            m_VolumeAtlas = null;
+            if (m_VolumeAtlas != null)
+            {
+                volumeAtlas.Release();
+                m_VolumeAtlas = null;
+            }
         }
     }
 }