* **Electron Version:**
  * 5.0.x
* **Operating System:**
  * macOS 10.14.5

### Expected Behavior
Electron does not crash at exit.

### Actual Behavior
The notifications are sent well, i use the close() function to avoid persistence in Notification Center. 

By doing this every time I quit the application I have a message that warns me that the application did not stop properly.

### To Reproduce
```javascript
const notif = new Notification({title: title, body: body, silent: true})
notif.on('click', () => appEvent.emit('show-window'))
notif.show()
setTimeout(() => notif.close(), 10000)
```
After sending notification(s), close application (app.quit()) and the error will appear.

To no longer have the error after quitting app, i do not use the function close () but my application generates a lot of notifications and I would have preferred to handle that.

shell/browser/api/atom_api_notification.cc
if (notification_)
    notification_->set_delegate(nullptr);
}
gin_helper::WrappableBase* Notification::New(gin_helper::ErrorThrower thrower,
                                             gin::Arguments* args) {
  if (!Browser::Get()->is_ready()) {
    thrower.ThrowError("Cannot create Notification before app is ready");
    return nullptr;
  }
  return new Notification(args);
}
base::string16 Notification::GetTitle() const {