Brooken with certain themes

Hi,
I’m seeing this error on my blog:

Warning: Missing argument 2 for HEADST4WP::title_filter() in /home/seowords/public_html/hanghimwithhispencopywriter.com/wp-content/plugins/headline-split-tester/headline-split-test.php on line 234

Here’s my blog: https://hanghimwithhispencopywriter.wordswordsseowords.com

I also see it’s broken on this blog:
https://www.ecoutere.com/portland-event-2nd-annual-ecouture-sustainable-fashion-show/?isalt=0