Game exits to main menu when it loses focus for too long

Build: Release_418100 (Windows 10 edition)

Summary:
Losing focus sends the game back to the main menu.

Impact:
Tabbing out of a work in progress means that the world has to be reopened almost every time, which is a problem when tabbing out is common.

Repro Steps:
1. Have the game window lose focus for around 5-10 seconds, either by clicking out or alt-tabbing.

Observed Results:
The game exits to the main menu, closing whatever is currently open.

Expected Results:
Nothing should happen.