Closing the initial setup with Alt+F4 bypasses the setup process and allows the user to continue without ever specifying an install directory.
The install path is then null, which causes countless problems within the launcher:

• Displays [LAUNCH] with no status messages, and with no game directory, this will cause a crash.
• Updating the game installs it to the root of the drive (C:\ or /)
• Many parsing errors from trying to use null instead of a valid path.
• etc.

Clicking the x causes the launcher to exit normally.

High priority (instead of release blocker) because this only affects non-Steam users (the launcher is not publicly released outside of Steam yet), and because there is a fairly low chance of this happening in the wild anyway.