White page on the browser

Hi,

I have a web site with WordPress 5.4.4; when I try to create a new page or when
when I try to create a new event (with Very simple Event List 12.1 version plugin)
I get a white page on my browser. I also tried with different browsers bur doesn,t work.
In a log file I have the following lines (I replaced some paths with dots) :

PHP Fatal error: Uncaught Error: Call to undefined function get_header() in ………./htdocs/wp-content/themes/hueman/index.php:1
Stack trace:
#0 {main}
thrown in ……../htdocs/wp-content/themes/hueman/index.php on line 1
[27-Nov-2020 09:09:37 UTC] PHP Fatal error: Uncaught ArgumentCountError: Too few arguments to function gutenberg_clear_synchronize_last_checks_after_delete(), 1 passed in ………./htdocs/wp-includes/class-wp-hook.php on line 287 and exactly 2 expected in ………./htdocs/wp-content/plugins/gutenberg/lib/templates-sync.php:169
Stack trace:
#0 ………./htdocs/wp-includes/class-wp-hook.php(287): gutenberg_clear_synchronize_last_checks_after_delete()
#1 ………./htdocs/wp-includes/class-wp-hook.php(311): WP_Hook->apply_filters()
#2 ………./htdocs/wp-includes/plugin.php(478): WP_Hook->do_action()
#3 ………./htdocs/wp-includes/post.php(3070): do_action()
#4 ………./htdocs/wp-includes/post.php(6990): wp_delete_post()
#5 ………./htdocs/wp-includes/class-wp-hook.php(287): wp_delete_auto_drafts()
#6 ………./htdocs/wp-content/plugins/gutenberg/lib/templates-sync.php on line 169

How can I resolve ?
Thank You.