Problem with plugin and theme

hello
The Hummingbird plugin on the Nimva Theme makes featured images dissapear.May i should mention that every article uses multi featured images up to five shown in a carousell.
A debug gives me the following result "Notice: Undefined variable: video_bg_render in /public/sites/www.imochique.com/wp-content/plugins/js_composer/include/templates/shortcodes/vc_row.php on line 234
" which i think has nothing to do with the plugin.
Support access is active.
Many thank for your help.
All the best
Dieter