Error message that changed the website

Hi,

i have an error message in wordpress dashboard:

Warning: Undefined array key “action” in /home/clients/7593e0463dbecc1ffde2a35a472629b3/web/wp-content/plugins/wp-user-manager/includes/filters.php on line 385

and everything seems not to be working also in line 385 is this:

if ( ‘switch_to_olduser’ === $query[‘action’] ) {
        $request_uri = isset( $_SERVER[‘REQUEST_URI’] ) ? wp_unslash( filter_input( INPUT_SERVER, ‘REQUEST_URI’ ) ) : ”;

        wp_safe_redirect( get_bloginfo( ‘wpurl’ ) . $request_uri );
        exit;
    }

    return $handler;
} );

Does anybody know how to solve this?

Thank you in advance!