I was trying to use signal-composer to get data from low-can and gps, so I tried as below:

• modify an application's config.xml file to require signal-composer.

  <feature name="urn:AGL:widget:required-api">								
   <param name="signal-composer" value="ws"/>					
  </feature>
  

• start the application and look at journalctl log.

With upper operation, I got the error log in log.txt.
Please help checking whether there are any errors existing.