Getting an error when testing out reg page

Hello,
I just created a registration page. I tested it out and got this error message. What can I do to fix it?

Warning: Illegal string offset ‘Message’ in /home/lakeca5/public_html/wp-content/plugins/front-end-only-users/Shortcodes/Insert_Register_Form.php on line 37

thanks,
Vickie

https://ww.wp.xz.cn/plugins/front-end-only-users/