Fatal Error when sending test

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My site is no longer sending emails. I’m getting this error

Fatal error: Uncaught Error: Call to a member function is_mailer_complete() on null in /home/mysite/public_html/wp-content/plugins/wp-mail-smtp/src/Admin/Pages/TestTab.php:105 Stack trace: #0 /home/mysite/public_html/wp-content/plugins/wp-mail-smtp/src/Admin/Area.php(552): WPMailSMTP\Admin\Pages\TestTab->display() #1 /home/mysite/public_html/wp-content/plugins/wp-mail-smtp/src/Admin/Area.php(533): WPMailSMTP\Admin\Area->display_current_tab_content() #2 /home/mysite/public_html/wp-content/plugins/wp-mail-smtp/src/Admin/Area.php(465): WPMailSMTP\Admin\Area->display_tabs() #3 /home/mysite/public_html/wp-includes/class-wp-hook.php(287): WPMailSMTP\Admin\Area->display(”) #4 /home/mysite/public_html/wp-includes/class-wp-hook.php(311): WP_Hook->apply_filters(”, Array) #5 /home/mysite/public_html/wp-includes/plugin.php(478): WP_Hook->do_action(Array) #6 /home/mysite/public_html/wp-admin/admin.php(254): do_action(‘toplevel_page_w…’) #7 {main} thrown in /home/mysite/public_html/wp-content/plugins/wp-mail-smtp/src/Admin/Pages/TestTab.php on line 105

Any idea what it’s referring to please?

Thanks