This message
Failed to load resource: the server responded with a status of 404 (Not Found) http://127.0.0.1:58889/http-services/ui-builder/web/unit_tests/utils/tes... "
is not an actual problem. Some code used for testing was left inside App Designer. Its presence or absence doesn't affect your app or the use of the XDK. You can safely ignore this message. We'll make sure it is not present in the next release.