Remove Update button from ContextualFormPanel

❌ Does not fix the issue — applied MR !510, rebuilt caches and configured a Tags referenced-entities block on the article Entity View Display: the Update button is gone and the preview updates on its own, but clicking "Add content" under the field now saves the empty nested source and the builder preview crashes with Error: Call to a member function getValue() on null until a source is picked. Without the MR the same click does not crash the preview, and the Update button was not needed there to get the preview updated.

Prerequisites

• Display Builder enabled on node.article.default with the default profile.
• Blocks › Referenced entities › "Tags (Taxonomy term) referenced by Tags", Source [Entity] ➜ [Field], Field Name, then "Add content" and leave the new Source on "- Select -".

Actual results

• MR !510: the preview shows the error above, thrown in DerivableContextSourceBase::getPropValue() (ui_patterns, line 106); picking [Field item] Text value recovers it.
• 1.0.x: no error; selecting [Field item] Text value updated the preview without clicking Update, so the use case in the issue summary did not reproduce.

Code review

• MR !510 at d9f6ce62, rebased on 1.0.x, pipeline green.
• Drupal.Ajax.prototype.success now triggers a save after every #ajax response, not only change events, so the "Add content" click persists an incomplete source.

Follow-up to comment #11 by @csakiistvan.

I added a fix in ApiController::update(). A new removeEmptySlotRows() method drops, before setSource(), any sources row with an empty source_id. The automatic save that fires after every #ajax response therefore no longer persists incomplete rows. The Update button removal and the JS are unchanged.

• Tests: two new Kernel tests in ContextualFormPanelRoundTripTest (testEmptySlotRowIsNotPersisted and testCompleteSlotRowIsPersisted). They fail without the fix and pass with it. The contextual_form, preview and textarea Playwright specs pass with the fix (13/13, Firefox only).
• Screenshots (before and after): on the "Test switch" block (Type: Component, Component: Test 1), in Slot 1 I set the Source to "- Select -" and reload. Before, the empty row is still stored after the reload. After, no row remains. The row shows as "Wysiwyg" because that is the select's default value; the stored source_id was empty.

What I could not verify:

• I did not reproduce the Call to a member function getValue() on null error, neither before nor after the fix. From the ui_patterns code, getSourcePlugins() only does an isset() check on source_id, so an empty value reaches createSourcePlugin(''), which returns NULL. But I did not execute that path. In my test setup the builder does not offer the entity_field source in that block's slot.
• I did not verify the summary's use case either (Tags, Entity → Field → Field item → Text value, Publish without the Update button).

@csakiistvan, could you share which component, context and steps you used to reach the nested source? With that I can reproduce it and confirm the fix covers the error.

Thanks @andres alvarez for the follow-up fix and the new Kernel tests.

❌ Does not fix the issue — applied MR !510, rebuilt caches and repeated the steps from comment #11 on the article Entity View Display: the getValue() on null crash is gone, but leaving the new nested Source on "- Select -" now shows Warning: Undefined array key 0 in DerivableContextSourceBase->getSourcePlugins() (ui_patterns, line 166) twice in the preview and as a message. Picking [Field item] Text value clears it and the preview updates without the Update button, so the empty-source path still needs handling.

Suggestion

Drop the key once no row is left, so ui_patterns takes its existing !isset($source_configuration['source_id']) early return (not tested here):

if ($key === 'sources') {
  $value = \array_filter($value, ...);
  if ($value === []) {
    unset($settings[$key]);
    continue;
  }
}

Alternatively, guard it in ui_patterns with !empty($sources['sources']) instead of isset().