Critical Error Activation

Hello. I’m leaving this in case you want to investigate. I’m afraid I can’t spend anymore time on this issue. I’ll edit my review as I should’ve posted it here. Thank you.

‘An error of type E_ERROR was caused in line 28 of the file /home/mruser/public_html/wp-content/plugins/critical-css-for-wp/includes/class-critical-css.php. 

Error message: Uncaught Error: Call to a member function is_preview_mode() on null in /home/mruser/public_html/wp-content/plugins/critical-css-for-wp/includes/class-critical-css.php:28
Stack trace:
#0 /home/mruser/public_html/wp-content/plugins/critical-css-for-wp/includes/class-critical-css.php(1362): Class_critical_css_for_wp->critical_hooks()
#1 /home/mruser/public_html/wp-content/plugins/critical-css-for-wp/critical-css-for-wp.php(34): require_once('/home/mruser...')
#2 /home/mruser/public_html/wp-settings.php(462): include_once('/home/mruser...')
#3 /home/mruser/public_html/wp-config.php(84): require_once('/home/mruser...')
#4 /home/mruser/public_html/wp-load.php(50): require_once('/home/mruser...')
#5 /home/mruser/public_html/wp-admin/admin.php(34): require_once('/home/mruser...')
#6 /home/mruser/public_html/wp-admin/plugins.php(10): require_once('/home/mruser...')
#7 {main}
  thrown