Editor crashes on quitting unity when project is building.

Steps to Repro:
1. Create a new project
2. Select "Build".
3. While building process, Select File > Quit.

Observe the crash

Expected Result: Build process should stop and unity should quit gracefully.

Actual Result: Editor crashes.

Reproducible on:
2018.2.0a8, 2018.2.0a1, 2018.1.0b13, 2018.1.0b2, 2017.3.0f3, 2017.2.0f3, 2017.1.0f3, 5.6.0f3.

Note: On 2018.1.0b1, editor crashes when platform is switched to WebGL.

Environment:
Occurring on Windows10, Windows 7.