PHP Recoverable fatal error

Using WP 5.1, You are stating compatible up to 4.9.9
When using AMP plugin trying to use paired mode (I think) I see in my error log the following:
[27-Feb-2019 10:38:41 UTC] PHP Recoverable fatal error: Object of class Closure could not be converted to string in /home/…/wp-content/plugins/widget-options/includes/widgets/extras.php on line 135

Then looking on github, it was reported before or something close to it.

https://github.com/phpbits/widget-options/issues/30