Steps to reproduce:
1. Create a new project and download HDRP package from the Package Manager
2. Set HDRP Render Pipeline Asset in Graphic settings
3. Select the Main camera in the scene
4. With mouse hover over Scene window

Results: Multiple exceptions are thrown in the Console

NullReferenceException: Object reference not set to an instance of an object
UnityEditor.Experimental.Rendering.HDPipeline.HDCameraEditor.SceneViewOverlay_Window (UnityEngine.GUIContent title, System.Action`2[T1,T2] sceneViewFunc, System.Int32 order, UnityEngine.Object target) (at Library/PackageCache/com.unity.render-pipelines.high-definition@5.2.3-preview/Editor/RenderPipeline/Camera/HDCameraEditor.Handlers.cs:61)
UnityEditor.Experimental.Rendering.HDPipeline.HDCameraEditor.OnSceneGUI () (at Library/PackageCache/com.unity.render-pipelines.high-definition@5.2.3-preview/Editor/RenderPipeline/Camera/HDCameraEditor.Handlers.cs:23)
System.Reflection.MonoMethod.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at <1aa022ba59ae4cfa86ec80b69d3d4fc2>:0)
Rethrow as TargetInvocationException: Exception has been thrown by the target of an invocation.

Reproduced with: 2019.2.0a1, 2019.1.0a14
Not reproducible with: 2019.1.0a13

Regression introduced in: 2019.1.0a14

Tested with HDRP packages: 5.2.3, 5.0.0, 4.6.0