Errors with debug true

Backtrace from warning 'Undefined index: enable_css' at /home/xxx/public_html/wp-content/plugins/widgets-on-pages/widgets_on_pages.php 289:

    /home/xxx/public_html/wp-settings.php 255 calling include_once()
    /home/xxx/public_html/wp-config.php 225 calling require_once()
    /home/xxx/public_html/wp-load.php 37 calling require_once()
    /home/xxx/public_html/wp-admin/admin.php 31 calling require_once()

Disabling all add css functionality on widgets_on_pages.php removes this error.

please fix.
thanks.

https://www.ads-software.com/plugins/widgets-on-pages/