Multiple category posts not working


Thanks for your amazing template!
I’m installing the home and I’m having problems with Multiple category posts. I have a warning: “/wp-content/themes/greatmag/widgets/widget-multiple-cats-posts.php on line 33”.
You can see it here at the bottom of the page:

Could you please help me with this issue?

I solved it. I was using a single category, I think that was the problem.