Sometimes when you instantiate Videoplayer element with drag & drop from components view it does not appear to have 'source' property. 

If you edit the QML with code editor right after you have instantiated it - this might be more likely to happen

Repro level 1/5. 

It might be that DS is doing something when you add the component so it present in the 2D view but property view is not ‘assembled’ yet?