New DefaultCompositionGraph is created every time on
-Deleting already assigned DefaultCompositionGraph
-Or Selecting None from Asset selector window.

Steps to repro:
1. Create a new HDRP template project
2. Windows > Render Pipeline > HD Render Pipeline Compositor.
3. Enable Compositor > Composition Graph.
4. Click on Asset selector Button > Assign to None.
OR
4. Delete the already assigned DefaultCompositionGraph

Actual Result:
New DefaultCompositionGraph created every time

Expected Result:
Composition Graph assigns to None.

Reproducible in:
2020.2.0a18 with Package version 9.0.0 preview.33

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

Environment:
Occurring on Windows 10 & Mac 10.15.