Error “The site is experiencing technical difficulties”

Hello i have problem with plugin. I have wordpress 5.2.4 and my plugin version is 1.2.7.

The home page doesn’t work and i receive this email

Howdy!

Since WordPress 5.2 there is a built-in feature that detects when a plugin or theme causes a fatal error on your site, and notifies you with this automated email.

In this case, WordPress caught an error with one of your plugins, Opal Hotel Room Booking.

First, visit your website (https://151450005.linuxzone94.grserver.gr/iros/) and check for any visible issues. Next, visit the page where the error was caught (https://151450005.linuxzone94.grserver.gr/iros/iros/wp-admin/post.php?post=12&action=edit) and check for any visible issues.

Please contact your host for assistance with investigating this issue further.

If your site appears broken and you can’t access your dashboard normally, WordPress now has a special “recovery mode”. This lets you safely login to your dashboard and investigate further.

https://151450005.linuxzone94.grserver.gr/iros/wp-login.php?action=enter_recovery_mode&rm_token=bqPX63VjycVDXBphODZiUd&rm_key=BC8IZ4SKgs2y0N2jMnPji2

To keep your site safe, this link will expire in 1 day. Don’t worry about that, though: a new link will be emailed to you if the error occurs again after it expires.

Error Details
=============
An error of type E_ERROR was caused in line 116 of the file /var/www/vhosts/151450005.linuxzone94.grserver.gr/httpdocs/iros/wp-content/plugins/opal-hotel-room-booking/includes/room/class-opalhotel-room.php. Error message: Uncaught Error: Call to a member function get_discount() on null in /var/www/vhosts/151450005.linuxzone94.grserver.gr/httpdocs/iros/wp-content/plugins/opal-hotel-room-booking/includes/room/class-opalhotel-room.php:116
Stack trace:
#0 /var/www/vhosts/151450005.linuxzone94.grserver.gr/httpdocs/iros/wp-content/themes/mazison/opalhotel/content-room-grid.php(15): OpalHotel_Room->has_discounts(1573137943)
#1 /var/www/vhosts/151450005.linuxzone94.grserver.gr/httpdocs/iros/wp-includes/template.php(724): require(‘/var/www/vhosts…’)
#2 /var/www/vhosts/151450005.linuxzone94.grserver.gr/httpdocs/iros/wp-content/plugins/opal-hotel-room-booking/includes/opalhotel-core-functions.php(738): load_template(‘/var/www/vhosts…’, false)
#3 /var/www/vhosts/151450005.linuxzone94.grserver.gr/httpdocs/iros/wp-content/plugins/opal-hotel-room-booking/includes/class-opalhotel-shortcodes.php(623): opalhotel_get_template_part(‘content-room’, ‘grid’)
#4 /var/www/vhosts/151450005.linuxzone94.grserver.gr/httpdocs/iros/wp-content/plugins/op