How to reproduce:
1. Create a new Project
2. In the Package manager, find and install the Lightweight render pipeline 3.0.0 and Shadergraph 3.0.0
3. Create a new PBR Shadergraph
4. Right-click the master node and click "Show Generated Code"
5. Observe the console

Expected result: The generated shader code is shown successfully
Actual result: Unity throws "Argument cannot be null" error in the console

Reproduced with: Shadergraph 3.1.0, Shadergraph 3.0.0
Not reproducible with: Shadergraph 2.0.8

Note: The bug reproduces with both HDRP and LWRP. "Show generated code" option works fine if the either the Shadergraph package or the Render Pipeline package is on the 2.0.8 version.