It needs an update from the theme authors but for now, I’ve just edited two theme files.
It turned out that there are two lines that caused the error and the fix is simple:
In
/cwp-megaresponsive/widgets/banner-widget/banner-widget.php
The line
parent::__construct('CWP_Add_banner', 'CWP - Add banner', $widget_ops);
has to be changed to
parent::__construct('CWP_Add_banner', 'CWP - Add banner', $widget_ops);
and in
/cwp-megaresponsive/widgets/facebook-like-box/fb-like-box.php
The line
$this->WP_Widget('CWP_FB_Like_Box', 'CWP - Facebook like box', $widget_ops);
has to be changed to
parent::__construct('CWP_FB_Like_Box', 'CWP - Facebook like box', $widget_ops);
🙂