I have the following method where it is suppose to get the first image that is displayed from the content.

public function FirstImage($content)
{
$dom = new DOMDocument();
$dom->loadHTML($content);

Debug::dump($content);

}
The wierd thing is when i do $dom->loadHTML I get the following error Empty string supplied as input, but as soon as I dump it I do get the correct data. How can I fix this problem ?

I also done $this->owner->Content which has the same thing.

You've provided no context to your function.
If $content is an empty string, then it's because you've passed an empty string as the parameter.

You've also not provided any context as to how it's called. Is this a controller function or something applied via Extension?

I cannot help you with this.