-ArgumentNullException error is thrown on entering in prefab mode after adding Custom Passes in Custom Pass Volume.
-NullReferenceException error is thrown on exiting from the prefab mode.

Steps to repro:
1. Create a new HDRP project.
2. GameObject > Volume > Custom Pass > Create its Prefab.
3. Hierarchy > Select Custom Pass > Inspector > Add CustomeClear Render Pass.
4. Enter in Prefab mode from Inspector.

Actual Result:
ArgumentNullException error is thrown in the console.

Expected Result:
No error in the console.

Reproducible in:
2020.2.0b7, 2020.2.0a20 with Package Version 10.0.0-preview.27.

Working Fine in:
2020.2.0b6 with Package version 9.0.0-preview.54.

Environment:
Occurring on Windows 10 & Mac 10.15.

Stacktrace:
ArgumentNullException: Value cannot be null.
Parameter name: key
System.Collections.Generic.Dictionary`2[TKey,TValue].FindEntry (TKey key) (at <9577ac7a62ef43179789031239ba8798>:0)
System.Collections.Generic.Dictionary`2[TKey,TValue].TryGetValue (TKey key, TValue& value) (at <9577ac7a62ef43179789031239ba8798>:0)
UnityEditor.Rendering.HighDefinition.CustomPassVolumeEditor.GetCustomPassDrawer (UnityEditor.SerializedProperty pass, UnityEngine.Rendering.HighDefinition.CustomPass reference, System.Int32 listIndex) (at library/packagecache/com.unity.render-pipelines.high-definition@10.0.0-preview.27/editor/renderpipeline/custompass/custompassvolumeeditor.cs:111)
UnityEditor.Rendering.HighDefinition.CustomPassVolumeEditor+<>c__DisplayClass18_0.<CreateReorderableList>b__2 (System.Int32 index) (at library/packagecache/com.unity.render-pipelines.high-definition@10.0.0-preview.27/editor/renderpipeline/custompass/custompassvolumeeditor.cs:211)
UnityEditorInternal.ReorderableList.CacheIfNeeded () (at <b01046f8b5314fae9e67f2e947544d2e>:0)
UnityEditorInternal.ReorderableList.GetListElementHeight () (at <b01046f8b5314fae9e67f2e947544d2e>:0)
UnityEditorInternal.ReorderableList.GetHeight () (at <b01046f8b5314fae9e67f2e947544d2e>:0)
UnityEditor.Rendering.HighDefinition.CustomPassVolumeEditor.DrawCustomPassReorderableList () (at library/packagecache/com.unity.render-pipelines.high-definition@10.0.0-preview.27/editor/renderpipeline/custompass/custompassvolumeeditor.cs:174)
UnityEditor.Rendering.HighDefinition.CustomPassVolumeEditor.OnInspectorGUI () (at library/packagecache/com.unity.render-pipelines.high-definition@10.0.0-preview.27/editor/renderpipeline/custompass/custompassvolumeeditor.cs:69)
UnityEditor.UIElements.InspectorElement+<>c__DisplayClass58_0.<CreateIMGUIInspectorFromEditor>b__0 () (at <fe25ae58492f48c395e59c49713b9c5c>:0)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr, Boolean&)

NullReferenceException: Object reference not set to an instance of an object
UnityEngine.Rendering.HighDefinition.CustomPassVolume.CleanupPasses () (at library/packagecache/com.unity.render-pipelines.high-definition@10.0.0-preview.27/runtime/renderpipeline/renderpass/custompass/custompassvolume.cs:148)
UnityEngine.Rendering.HighDefinition.CustomPassVolume.OnDestroy () (at library/packagecache/com.unity.render-pipelines.high-definition@10.0.0-preview.27/runtime/renderpipeline/renderpass/custompass/custompassvolume.cs:80)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr, Boolean&)