Editor crashes if user quits while package manager is updating a package.

Steps to Reproduce:
1. Create a new project.
2. Open package manager and update any package ( say : Package Manager UI)
3. While its getting updated, quit Editor ( File Menu >> Quit)

Observe the crash. (Attached screenshot with error message, also find attached video to reproduce issue)

Reproducible On: 2018.2.0a1

Works Fine On: 2018.1.0b7, 2018.1.0b6

Notes:
- Appears always.
- Appears on both Mac and Windows.