Magento 2.1.1 Admin content pages not loading handle not registered
<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>