Not disposing APV stuff when the Lighting window closes and a bake is running

The purpose of this PR is to avoid a memory leak and null pointer references when the Lighting window closes while APV is baking

Author: belgaard

Packages/com.unity.render-pipelines.core/Editor/Lighting/ProbeVolume/ProbeVolumeLightingTab.cs

@@ -206,7 +206,10 @@ public override void OnDisable()
 
             EditorSceneManager.sceneOpened -= OnSceneOpened;
 
-            AdaptiveProbeVolumes.Dispose();
+            // We keep allocated acceleration structures while the Lighting window is open in order to make subsequent bakes faster, but when the window closes we dispose of them
+            // Unless a bake is running, in which case we leave disposing to CleanBakeData() 
+            if (!AdaptiveProbeVolumes.isRunning && !Lightmapping.isRunning)
+                AdaptiveProbeVolumes.Dispose();
         }
 
         #region On GUI