Conflict with FIFU plugin

Hi, guys!

This is Marcel, developer of Featured Image from URL plugin. First, thanks for keeping the integration.

Today an user reported this issue:
https://ww.wp.xz.cn/support/topic/how-to-fix-fatel-error/

There is a fatal error when he tries to enable FIFU after AMP. But if he enables FIFU before AMP, all are okay.

I believe it happens because you are checking if the function fifu_amp_url() exists.

if( !function_exists( 'fifu_amp_url' ) ) {
        function fifu_amp_url($url, $width, $height) {

A better way to do that would be checking if FIFU is installed, because the function fifu_amp_url() also exists in FIFU code. Thanks!

The page I need help with: [log in to see the link]