Steps to repro:

1. Open Unity;
2. Create new project and scene;
3. Enter Play mode;
4. Go to File -> Build Settings;
5. Select PC, Mac & Linux Standalone platform;
6. Click Build;
7. Observe the console.

Actual result:

'Callback unregistration failed. Callback not registered.
UnityEditor.HostView:OnGUI()'

error is thrown in the console after the build finishes. See attached gif video and actual.png screenshot.

Regression introduced in 5.4.

Expected result:
Errors should not be thrown when building in Play mode. In this case, Play mode should just be stopped and build should proceed without errors. You can find expected behaviour in 5.3.2f1.

Notes:
- Reproducible in: 5.4.0b6, 5.4.0b4, 5.4.0b4;
- Not reproducible in: 5.3.2f1, 5.3.1f1;
- Reproducible on both Windows 10 and OSX 10.11.3;
- Reproducible for both Build and Build&Run features.