Fix error on quit caused by Electron race issue

[whitecrownclown]

Fixes #809

[sindresorhus]

@whitecrownclown Are you able to reproduce the issue?

[sindresorhus]

The issue makes no sense as the windows are not supposed to be quit until after the before-quit event is called:

Emitted before the application starts closing its windows. Calling event.preventDefault() will prevent the default behaviour, which is terminating the application.
- https://electronjs.org/docs/api/app#event-before-quit

[whitecrownclown]

@sindresorhus No, I couldn't reproduce this issue and I agree, it doesn't make any sense. I thought there may be an electron issue which could cause it but I couldn't find anything related.

Edit: The PR obviously isn't a "real" fix, at least from what we can tell from the electron docs, I'm ok with either merging or closing it. Thanks!

[sindresorhus]

Merging this as more people are experiencing the issue...