title: Magento 2.1.1 Admin content pages not loading handle not registered
body: <p>In my Magento 2.1.1 shop with 2 custom modules some of my admin content pages are not loading.
  (Widget, theme and shedule are loading fine - others not :-( )</p>
  
  <p>For all of these I get "Not registered handle" error:
  Not registered handle cms_page_listing_data_source
  Not registered handle cms_block_listing_data_source
  Not registered handle design_config_listing_data_source</p>
  
  <p>While all of these do exist in the xml files.
  app/vendor/magento/module-cms/view/adminhtml/ui_component/cms_page_listing.xml
  ...</p>
  
  <p>Seems like this first occorred when I added the "demo" store option to the shop ... but I'm not sure.</p>
  
  <p>Any help would be great!</p>
  
  <p>Thanks
  Stijn</p>