Editor crashes on selecting "reset packages to default" from "Help" menu using templates.

Steps to Reproduce:
1. Create a new project using templates (3D or lightweight for example)
2. Create a new scene.
3. Go to Window > Package Manager.
4. Go to Help > Reset Packages to default.
5. Click on Cancel Button.

Actual Result: Observe Crash

Reproducible on: 2018.2.0a5, 2018.1.0b11, 2018.1.0b6, 2018.1.0b5

Environment
Occurs on Windows only.