Error: Call to a member function addError() on null

Problem/Motivation

Table creation fails after "save" in D8, and also in D9.

Proposed resolution

- Support Messenger system correctly.

Remaining tasks

1. Inject *messenger* service as a dependency for data in data.services.yml per https://www.drupal.org/node/2774931
1. ... tbd

Original report by [username]

I'm 90% sure #3088972: Drupal 9 plan was committed without testing and it's fairly easy to fix.

Upon submitting the form to add/edit a data table, you get a fatal error saying "unable to call addError() on NULL" or somesuch, which is because $this->messenger is undefined. So I guess it is up to the form class to add a constructor which injects the messenger object.

I tested this on Core 8.7 / 8.8 / 8.9 before realizing this. Since this form is the only one that works so far (?) in the D8 version, I guess that counts as "the module is broken".

(@ Anyone: feel free to fix. I was just giving the module a spin, to check if I should port some of the D7 patches I'm uploading, to D8 too. I think I have my answer.)