FileWidget inside subform can't find its values

Wondering if #2653106: Introduce SubformInterface or #2537732: PluginFormInterface must have access to the complete $form_state (introduce SubFormState for embedded forms) might solve this, and in the end paragraphs should take advantage of it.

This also affects the field collection module.

I agree with the proposed fix. The problem seems to be a mixup between #parents and #array_parents. For example, for field collections, the following change will also work:

    $parents = $element['#parents'];
    $values = NestedArray::getValue($form_state->getValues(), $parents);

    // Add the following line:
    $parents = $element['#array_parents'];

    array_pop($parents);
    $current = count(Element::children(NestedArray::getValue($form, $parents))) - 1;

The original patch does not work, because it tries to get the child elements from form state values, which is impossible.

This patch tries to stay as close as possible to the original solution (because I did not want to dig through the array structures, not because I think we necessarily must). It also fixes the test failures. It seems the original error handling and its test coverage was not very good, unless I missed something.

The patch works in my situation:
- Node with Paragraph field of Paragraph type with (text and) image field with 3 values.

Initial work around was to make the image field cardinality unlimited.

Tested manually and works fine with the patch! Let's see what happens with the test only patch. :)

Cool, then this fixes the problem I think

I tested #8 on 8.1.3 and it works as well. I reviewed the code and it and looks good to me.

Thanks for providing a test-only patch!

Due to a quirk of the testing system, we need to make sure that the actual patch is the final attachment (otherwise it does not get automatically retested). So reuploading that, and saving issue credit.

Back to RTBC as per #13.

Committed/pushed to 8.2.x and cherry-picked to 8.1.x. Thanks!