help:resource did not handle connector request

I use the code to create a resource connector:

ResourceReference resr=ResourceReference.create(source, this,“src”);

and in the GWT client connector,I use resr.getURL() method,

the URL show in the web elements is that:

but the URL is unavailable,show that:

二月 11, 2019 1:45:13 下午 com.vaadin.server.ConnectorResourceHandler error
警告: VideoPlayView (153) did not handle connector request for src/VID_20190119_094502.mp4

how could i do? thanks!