Custom Blocks not importable

[omelhoro]

Hi all, I'm trying to follow the instruction of integrating customer blocks into the editor, but it throws always some error. I setup the editor following https://docs.unlayer.com/docs/getting-started and also set a user id. The save button appears for rows and when I click it, there is a request to backend to save it (status 200, "success": true). Now when I reload the site the custom block doesn't appear again.

Also when I tried to manually add the content (the data prop) of the POST /blocks request into the blocks property it throws an error:

Uncaught TypeError: Cannot read properties of undefined (reading 'forEach')

and it points to this code:

return null == o || o.rows.forEach((function(t) {
                    var r = e.rows[t]
                      , n = [];
                    r.columns.forEach((function(t) {

where it can't iterate over r.columns because r is undefined.

I created also a custom block in the unlayer UI but that also doesn't seem to appear in the editor when I set my project id.

So how you guys make the custom blocks work?