Hi Offi & Elanora
I also experienced this issue, several times.
The first time was when I saved the entire page in a new name, with the intent to edit it offline before publishing.
But when I published it and tried to see it live, it failed with "Attempt to assign property "row_id" on null"
By hiding all of the sections one at a time, I was able to determine the error was coming from a contact form section. For some reason, the details in the contact form were blank, and entering details in the contact form fixed this for me.
I did try the language add-on but it was disabled, and enabling it didn't change the issue.
Maybe this will help future me's running into this issue.