ArgumentNullException error is thrown on saving HDRP Shader Graph after removing Graphic compositor assets.

Steps to repro:
1. Create a new HDRP template project
2. Windows > Render Pipeline > Graphics Compositor >Enable Compositor.
3. Disable Compositor > Remove Compositor from Scene.
4. Delete folder Asset/ Compositor.
5. ProjectWindow > Click on "+" > Shader > HDRP > StackLit Graph.
6. Open Shader Graph > Save Asset.

Actual Result:
ArgumentNullException error is thrown in the console.

Expected Result:
No errors in the console.

Reproducible in:
2020.2.0a20, 2020.1.2f1, 2020.1.0b13 with Package version 10.0.0-preview.27, 9.0.0 preview.33.

Note: HDRP Compositor window introduced in 9.0.0 preview.33.

Environment:
Occurring on Windows 10 & Mac 10.15.