public function onShutdown()
    {
        // If the instance was unset, then we shouldn't run.
        if (self::$instance == null) {
            return;
        }

        // This will help prevent multiple calls to this, incase the shutdown handler was declared
        // multiple times. This only should occur in unit tests, when the handlers are created
        // and removed repeatedly. As we cannot remove shutdown handlers, this prevents us from
        // calling it 1000 times at the end.
        self::$instance = null;

        // Get the last error if there was one, if not, let's get out of here.
        if (!$error = error_get_last()) {
            return;
        }

        // Don't notify on warning if not configured to.
        if (!$this->shouldNotifyError($error['type'], $error['message'], $error['file'], $error['line'])) {
            return;
        }

        // Build a fake backtrace, so we at least can show where we came from.
        $backtrace = array(
            array(
                'file' => $error['file'],
                'line' => $error['line'],
                'function' => '',
                'args' => array(),
            )
        );

        $this->airbrakeClient->notifyOnError('[Improper Shutdown] '.$error['message'], $backtrace);
    }