Add a warning if Ros is not initialized

[Daenyx]

Describe the bug
I got no display of the image transport on Qt

Expected behavior
I excepted the topic image transport to display

Desktop (please complete the following information):

• OS: Ubuntu 18.04
• ROS: Melodic
• Qt: 5.9.5

Additional context

• When i launch my app on Qt i see the topic it tried to subscribe ("/usb_cam/image_raw/compressed") but nothing appear on the app but usb_cam package topic works perfectly

Code

Item {
  x: 440
  y: 261
  width: 400
  height: 400

  ImageTransportSubscriber {
    id: imageSubscriber
    topic: "/usb_cam/image_raw"
    throttleRate: 0.2 // 1 frame every 5 seconds
    defaultTransport: "compressed"
  }

  VideoOutput {
      width: 400
      height: 400
      source: imageSubscriber
  }
}

[Daenyx]

Finally it worked i had to add
Component.onCompleted: { Ros.init("image_subscriber") }
at the end.

[StefanFabian]

Glad you figured it out on your own. It's documented on the Ros singleton page but I think it makes sense to add a warning if after 5 seconds Ros was not initialized.