RSS Feed Widget Problem

Helo i have a problem with RSS feed widget at first when i try to add url i got A feed with an invalid mime type may fall victim to this error, or SimplePie was unable to auto-discover it.. Use force_feed() if you are certain this URL is a real feed. so i add the following code in functions.php

add_action('wp_feed_options', 'force_feed', 10, 1);
function force_feed($feed) {
	$feed->force_feed(true);
}

and i tried to add the URL link now and i got this error

RSS Error: This XML document is invalid, likely due to invalid characters. XML error: junk after document element at line 92, column 0

i tried on another theme it's the same problem