Error in combination wit Zerif Pro theme

Including the short code [kbe_knowledgebase] in de short code section of Zerif Pro gives this error:

Fatal error: Uncaught Error: Call to a member function get_var() on null in D:\home\site\wwwroot\wp-content\plugins\wp-knowledgebase\template\kbe_knowledgebase.php:78 Stack trace: #0 D:\home\site\wwwroot\wp-content\plugins\wp-knowledgebase\wp-knowledgebase.php(541): require() #1 D:\home\site\wwwroot\wp-includes\shortcodes.php(326): kbe_shortcode(”, ”, ‘kbe_knowledgeba…’) #2 [internal function]: do_shortcode_tag(Array) #3 D:\home\site\wwwroot\wp-includes\shortcodes.php(223): preg_replace_callback(‘/\\[(\\[?)(kbe_kn…’, ‘do_shortcode_ta…’, ‘[kbe_knowledgeb…’) #4 D:\home\site\wwwroot\wp-content\themes\zerif-pro\sections\shortcodes.php(33): do_shortcode(‘[kbe_knowledgeb…’) #5 D:\home\site\wwwroot\wp-includes\template.php(574): require(‘D:\\home\\site\\ww…’) #6 D:\home\site\wwwroot\wp-includes\template.php(531): load_template(‘D:\\home\\site\\ww…’, false) #7 D:\home\site\wwwroot\wp-includes\general-template.php(167): locate_template(Array, true, false) #8 D:\home\site\wwwroot\wp-content\themes\zerif-pro\front-pa in D:\home\site\wwwroot\wp-content\plugins\wp-knowledgebase\template\kbe_knowledgebase.php on line 78

If I use the same short code in a page (with the same theme) the knowledge base is loaded correctly.

What can I do about this error?