Error on Ajax request

Fatal error: Uncaught TypeError: call_user_func_array(): Argument #1 ($callback) must be a valid callback, function "fvm_end_buffer" not found or invalid function name in ...\wp-includes\class-wp-hook.php:307 Stack trace: #0 ...\wp-includes\class-wp-hook.php(331): WP_Hook->apply_filters('', Array) #1 ...\wp-includes\plugin.php(476): WP_Hook->do_action(Array) #2 ...\wp-includes\load.php(1102): do_action('shutdown') #3 [internal function]: shutdown_action_hook() #4 {main} thrown in ...\wp-includes\class-wp-hook.php on line 307