Update: The crash no longer occurs, but terrain material is not properly upgraded. Need to find out the following:
- Should Upgrade Project Materials work for terrain material? -> yes!
- Should Upgrade Project Materials automatically regenerate the materials used in detail prefabs? -> yes!
- Should Upgrade Project Materials automatically set Materials Location to "Use Embedded Materials"? -> if it's required to get materials working!

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When trying to upgrade a project to URP, clicking 'Regenerate Materials' on a tree in the terrain will crash the editor. (Project GitHub link in comments.)

1. Clone the project linked in the comments.
2. Open project in editor. Upgrade to 2019.3.0f2.
3. Open the scene 'TerrainLighting'.
4. Download the Universal RP Package. (Window -> Package Manager)
5. Create a new URP Pipeline Asset. (Assets -> Create -> Rendering -> Universal Render Pipeline -> Pipeline Asset)
6. Set this new Pipeline Asset at the active SRP in Project Settings -> Graphics.
7. Try to upgrade to URP materials. (Edit -> Render Pipeline -> Universal Render Pipeline -> Upgrade Project Materials to UniversalRP Materials)
- This doesn't do much.
8. In project view navigate to the tree used in the terrain by going to Assets -> Resourses -> SpeedTree -> SpeedTreeMain -> Free_SpeedTrees -> Broadleaf_Desktop
9. Click on 'Broadleaf_Desktop' to view in Inspector.
10. In Inspector view, click on the 'Materials' tab and click 'Regenerate Materials'.

Expected Outcome: Materials are regenerated. No issues.
Actual Outcome: Project crashes.

Reproducible when upgrading project to 2019.3.0f2, 2019.3.0b6, 2019.3.0b4, 2019.3.0b1 and 2019.3.0a12 with URP version 7.1.6 - 7.0.1
Not reproducible when upgrading project to 2019.3.0a10 and 2019.3.0a11 with URP version 7.0.0.

Note:
- Tried a project from scratch with out upgrade with terrain and a speedtree, upgraded to URP. Did not crash.
- Tested primarily on Windows. Did not crash on Mac in 2019.3.0f2.
- This crash occurred when trying to regenerate materials on a seperate terrain project.
- Different things happen when you try this set of steps with HDRP.
- After the crash, you can re-open the project and repeat those steps with no crash.
- If you re-clone the project and follow the steps from scratch, it will crash.
- GIF demonstrating moment of crash and video of steps leading to crash attached.