default featured image and image size

Hi dear Support,

I’m building an add-on that has a problem with the default image
I followed this link https://github.com/…/blob/m…/docs/content/controls/_media.md
Because I use those
namespace Widgets;
I use instead of
namespace Elementor;
I having trouble in this section, Can you guide me to make up for the deficiencies that are in the add-on for default image?

'default' => [
     'url' => Utils :: get_placeholder_image_src (),
],

Also one more thing, I couldn’t be included theme image cropper size I think the below code is correct; but, I returned to above issue
Group_Control_Image_Size::get_type(),
I appreciate your help.

https://scontent-lga3-1.xx.fbcdn.net/v/t1.0-9/27332616_1545075135575062_6494311033501010903_n.jpg?oh=62d55ac76388cb51d0c5207d5373640f&oe=5B230392

Please Note: I don’t have any Error in back-end and front-end.

Many Thanks