* **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
v8::Local<v8::FunctionTemplate> prototype) {
  prototype->SetClassName(gin::StringToV8(isolate, "Notification"));
  gin_helper::Destroyable::MakeDestroyable(isolate, prototype);
  gin_helper::ObjectTemplateBuilder(isolate, prototype->PrototypeTemplate())
      .SetMethod("show", &Notification::Show)
      .SetMethod("close", &Notification::Close)
      .SetProperty("title", &Notification::GetTitle, &Notification::SetTitle)
      .SetProperty("subtitle", &Notification::GetSubtitle,
                   &Notification::SetSubtitle)
      .SetProperty("body", &Notification::GetBody, &Notification::SetBody)
      .SetProperty("silent", &Notification::GetSilent, &Notification::SetSilent)
      .SetProperty("hasReply", &Notification::GetHasReply,
                   &Notification::SetHasReply)
      .SetProperty("timeoutType", &Notification::GetTimeoutType,
                   &Notification::SetTimeoutType)
      .SetProperty("replyPlaceholder", &Notification::GetReplyPlaceholder,
                   &Notification::SetReplyPlaceholder)
      .SetProperty("urgency", &Notification::GetUrgency,
                   &Notification::SetUrgency)
      .SetProperty("sound", &Notification::GetSound, &Notification::SetSound)
      .SetProperty("actions", &Notification::GetActions,
                   &Notification::SetActions)
      .SetProperty("closeButtonText", &Notification::GetCloseButtonText,
                   &Notification::SetCloseButtonText);
}
}  
}  
namespace {
using electron::api::Notification;
void Initialize(v8::Local<v8::Object> exports,