Error related to blueprint causes Grav / Admin to crash

[yankl]

I am getting the follow error:

TypeError
Argument 1 passed to Grav\Common\Data\BlueprintSchema::processFormRecursive() must be of the type array or null, string given, called in C:\laragon\www\grav-frish\system\src\Grav\Common\Data\BlueprintSchema.php on line 320

Here is how I can reproduce this error on a relatively fresh install with Quark theme.
In quark/templates/ I add simple test.html.twig:

{% extends 'default.html.twig' %}

in user/pages/03.testbug I add test.md:

---
title: test
pick: BPY_1.1.1_Read_01.mp3
content_fallback:
    -
        key: bin
        value: BPY_1.1.1_Read_b_1.mp3
---

Finally, I add test.yaml to quark/blueprints/:

title: Oysyes Blueprint
'@extends':
    type: default
    context: blueprints://pages

form:
  fields:
    tabs:
      type: tabs
      active: 1
      
      fields:
        content:
          fields:
            header.letters:
              type: list
              label: Letters
              min: 4
              max: 4
              
              fields:
                .letter_itself:
                  type: list
                  label: Letter Audio
                  max: 2
                  
                  fields: 
                    .letter:
                      type: text
                      label: Letter
                    .file:
                      type: filepicker
                      folder: 'self@'
                      accept:
                        - .mp3
                      label: Audio
                
                .words:
                  type: list
                  label: Words
                  
                  fields:
                    .word:
                      type: text
                      label: Word
                     
                    .file:
                      type: filepicker
                      folder: 'self@'
                      accept:
                        - .mp3
                      label: Audio
                      
            .words_you_know:
              type: list
              label: Known Words
              
              fields:
                .word:
                  type: text
                  label: Word
                 
                .file:
                  type: filepicker
                  folder: 'self@'
                  accept:
                    - .mp3
                  label: Audio
            
            .words_to_read:
              type: list
              label: Words to Read
              
              fields:
                .word:
                  type: text
                  label: Word
                 
                .file:
                  type: filepicker
                  folder: 'self@'
                  accept:
                    - .mp3
                  label: Audio

Then, in .../admin/pages/testbug/mode:normal, the blueprint seems to work correctly in terms of the form created. But when I click Save, I get the mentioned error:

I'm sure this can be further simplified but that's what I have for now. I made an issue here instead of in Admin plugin, because the error is coming from Grav\Common\Data\BlueprintSchema and not from the Admin plugin.

Grav v1.7.15 - Admin v1.10.15

[mahagr]

.words_you_know and .words_to_read are in top level and should have header prefix.

[yankl]

Indeed!! That fixes it. I just started learning how to compose blueprints, and missed that.
Thanks so much!

Still, I'll leave the issue open in case the developers would like to consider presenting a friendlier error message in such a case, that would help point to the problem.

[mahagr]

I agree with the better error message (or just accept it) if the dot is used on the main level.

[mahagr]

The issue here is that you have a tab named content, and .words_to_read is nested inside it. This overrides the main content with an array.