Critical error – blocking login

Hello,
this weekend I received an admin email from my WordPress installation which was telling me that there’s an error with CF7 Smart Grid Design Extension plugin (version 4.12.12).
The error is:

E_ERROR in line 866 of file /wp-content/plugins/cf7-grid-layout/admin/class-cf7-grid-layout-admin.php. Error Message: Uncaught Error: Class 'WPCF7_ContactForm' not found in /wp-content/plugins/cf7-grid-layout/admin/class-cf7-grid-layout-admin.php:866
Stack trace:
#0 /wp-includes/class-wp-hook.php(309): Cf7_Grid_Layout_Admin->delete_post(1023)
#1 /wp-includes/class-wp-hook.php(331): WP_Hook->apply_filters(NULL, Array)
#2 /wp-includes/plugin.php(474): WP_Hook->do_action(Array)
#3 /wp-includes/post.php(3409): do_action('before_delete_p...', 1023, Object(WP_Post))
#4 /wp-content/plugins/contact-form-7-paypal-add-on-pro/includes/admin/files.php(24): wp_delete_post(1023, true)
#5 /wp-content/plugins/contact-form-7-paypal-add-on-pro/paypal.php(141): include_once('/home/trsdyouq/...')
#6 /public_html/wp-settings.php(418): include_once('/home/trsdyouq/...')
#7 /public_html/wp-config.php(91): require_once('/home/trsdyouq/...')

It was impossible to login in backend: in the /wp-admin page there was a critical error and the login form didn’t appear. I had to force via FTP the deactivation of CF7 Smart Grid Design Extension plugin. After that I was able again to login. I reactivate the plugin and everything seemed to work.

But I would like to understand with you what caused the error (because maybe it could happen again).

Wait for your kind response
Best Regards