Machine name field throws notices if before source field.

Should have used #after_build instead of #process.

I see, let's try this one instead.

hm... so it actually would have to be a #pre_render, but still needs access to $form_state, and that callback would have to be invoked before either of both elements is attempted to be rendered... very tricky. :-|

Subscribing. Happened to me too; confusing until I saw this issue.

If you manually add the '#id' to the machine name and the field in 'source', the error is gone.

In D7 this also leads to a JS error: The empty #id is put into Drupal.settings.machineName and the behavior then tries to do a $('#', …) which throws a syntax error.

Still having this issue in a config entity form (8.3.4)

// Works 
$form['label'] = [
      '#type' => 'textfield',
      '#title' => $this->t('Label'),
      '#maxlength' => 255,
      '#default_value' => $fee_info->label(),
      '#description' => $this->t("Label for the Fee info."),
      '#required' => TRUE,
      '#weight' => -5,
    ];
// Never ever ever put this field at the top of  the form ¯\_(ツ)_/¯.
    $form['id'] = [
      '#title' => $this->t('FEE ID'),
      '#type' => 'machine_name',
      '#default_value' => $fee_info->id(),
      '#machine_name' => [
        'exists' => '\Drupal\dpe_fee_data\Entity\FeeInfo::load',
      ],
      '#disabled' => !$fee_info->isNew(),
      '#field_prefix' => 'FEE-',
      '#weight' => -10,
    ];

but if you put the machine_name field above the you get the "label" notice:
"Notice: Undefined index: #id in Drupal\Core\Render\Element\MachineName::processMachineName() (line 174 of core/lib/Drupal/Core/Render/Element/MachineName.php). "

I don't see any realistic approach to fixing this. I think we need to inform the developer that they must define the form elements in the correct order.

I've added a new field in the custom module through the yml file for the commerce_store entity which doesn't implement field api.
Despite the fact that I've added wight property into core.entity_form_display.commerce_store.online.default.yml it doesn't affect anything and I still see this error, however, the machine_name field is placed after source field.
Unfortunately, I don't see any approach to change the render order in this case.
Maybe someone knows how to fix it?

I think #17 is a different problem. I agree with #14 on this being really hard to solve in a good way and that this is a good fix. We should get a test for this as well, to make sure we don't regress the new behavior.

Here is a test for the new thrown exception. I've modified the message a bit to specify that you can also set an id on the source element to work around the problem.

The test-only patch test is a little different than the one with the fix but it illustrates the notice that gets thrown.

fix the cs fail

Fix the test

+++ b/core/lib/Drupal/Core/Render/Element/MachineName.php
@@ -183,6 +184,13 @@ public static function processMachineName(&$element, FormStateInterface $form_st
+      throw new \LogicException(sprintf('The machine name element "%s" is defined before the source element "%s", it must be defined after or the source element must specify an id.', $element_parents, $source_parents));

Is this always true? Would this exception also be thrown if the source element was missing from the form?

@joachim there is a guard statement for that before reaching this logic

    $source = NestedArray::getValue($form_state->getCompleteForm(), $element['#machine_name']['source'], $key_exists);
    if (!$key_exists) {
      return $element;
    }

So no, it won't be thrown in that case.

Thanks! In which case, happy to say RTBC.

Committed and pushed 25718d36d4 to 9.3.x and 4ba14b66af to 9.2.x. Thanks!