Autopost creates fatal error

[SunDancerGE]

So since the latest WP update every time I post a new article or just hit "preview" WP exits with a fatal error.

[20-Jan-2022 10:50:47 UTC] PHP Fatal error:  Uncaught TypeError: sizeof(): Argument #1 ($value) must be of type Countable|array, bool given in /mydomain.tld/www/wp-content/plugins/autopost-to-mastodon/mastodon_autopost.php:545
Stack trace:
#0 /mydomain.tld/www/wp-content/plugins/autopost-to-mastodon/mastodon_autopost.php(324): autopostToMastodon->getTootFromTemplate()
#1 /mydomain.tld/www/wp-includes/class-wp-hook.php(305): autopostToMastodon->toot_post()
#2 /mydomain.tld/www/wp-includes/class-wp-hook.php(327): WP_Hook->apply_filters()
#3 /mydomain.tld/www/wp-includes/plugin.php(470): WP_Hook->do_action()
#4 /mydomain.tld/www/wp-includes/post.php(4468): do_action()
#5 /mydomain.tld/www/wp-includes/revision.php(334): wp_insert_post()
#6 /mydomain.tld/www/wp-includes/revision.php(190): _wp_put_post_revision()
#7 /mydomain.tld/www/wp-includes/class-wp-hook.php(305): wp_save_post_revision()
#8 /mydomain.tld/www/wp-includes/class-wp-hook.php(327): WP_Hook->apply_filters()
#9 /mydomain.tld/www/wp-includes/plugin.php(470): WP_Hook->do_action()
#10 /mydomain.tld/www/wp-includes/post.php(4442): do_action()
#11 /mydomain.tld/www/wp-includes/post.php(4570): wp_insert_post()
#12 /mydomain.tld/www/wp-admin/includes/post.php(423): wp_update_post()
#13 /mydomain.tld/www/wp-admin/includes/post.php(1918): edit_post()
#14 /mydomain.tld/www/wp-admin/post.php(336): post_preview()
#15 {main}
  thrown in /mydomain.tld/www/wp-content/plugins/autopost-to-mastodon/mastodon_autopost.php on line 545

Any idea how to fix that?