When I googled for this error, I found numerous hits mentioning the error message appearing during a migration, even stopping it. While I have done a migration from Drupal 7 to 9 that ran smoothly, this is during normal running of the now Drupal 9-based site.
E.g. I create a new content. As soon as I select the content type and the content form appears, I get this error. So it is probably not related to text formats.
I have no idea how to provide the missing filter plugin. I searched a database dump and found several occurrences of "filter_null". Most notably in the tables "config", "key_value" and "key_value_expire"(!).
But I did not find "filter_null" or any other "filter_<something"> in the Drupal user interface. I did find code related to it, most notably the file
Can anyone tell me the trick to provide that filter? I'm still running 9.3.7, installing 9.3.8 is next on my agenda. But it does not look like there is any changed that relates to this error.