Preview Post Error: Class does not have a method

Upon clicking the button “Preview Post” on our website, it takes us to a blank webpage and displays this error:

Warning: call_user_func_array() expects parameter 1 to be a valid callback, class ‘RecentPostsWidget’ does not have a method ‘flush_widget_cache’ in /var/sites/s/steponefinance.co.uk/public_html/wp-includes/plugin.php on line 525

Warning: Cannot modify header information – headers already sent by (output started at /var/sites/s/steponefinance.co.uk/public_html/wp-includes/plugin.php:525) in /var/sites/s/steponefinance.co.uk/public_html/wp-includes/pluggable.php on line 1171

We are fully up to date, including plugins. any help would be greatly appreciated.