Issue #2193237 by czigor, amateescu, barraponto, revagomes, rodrigo panchiniak...

Issue #2193237 by czigor, amateescu, barraponto, revagomes, rodrigo panchiniak fernandes: Warning because of empty value at array_flip().

core/modules/taxonomy/lib/Drupal/taxonomy/Plugin/Field/FieldWidget/TaxonomyAutocompleteWidget.php

@@ -71,8 +71,10 @@ public function settingsSummary() {
    */
   public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, array &$form_state) {
     $tags = array();
-    foreach ($items as $item) {
-      $tags[] = isset($item->entity) ? $item->entity : entity_load('taxonomy_term', $item->target_id);
+    if (!$items->isEmpty()) {
+      foreach ($items as $item) {
+        $tags[] = isset($item->entity) ? $item->entity : entity_load('taxonomy_term', $item->target_id);
+      }
     }
     $element += array(
       '#type' => 'textfield',

core/modules/taxonomy/lib/Drupal/taxonomy/Tests/TermFieldTest.php

@@ -17,7 +17,7 @@ class TermFieldTest extends TaxonomyTestBase {
    *
    * @var array
    */
-  public static $modules = array('entity_test');
+  public static $modules = array('entity_test', 'field_ui');
 
   protected $instance;
   protected $vocabulary;
@@ -37,6 +37,7 @@ function setUp() {
       'view test entity',
       'administer entity_test content',
       'administer taxonomy',
+      'administer entity_test fields',
     ));
     $this->drupalLogin($web_user);
     $this->vocabulary = $this->createVocabulary();
@@ -128,6 +129,20 @@ function testTaxonomyTermFieldWidgets() {
     $this->assertNoFieldByName($this->field_name, '', 'Widget is not displayed.');
   }
 
+  /**
+   * No php error message on the field setting page for autocomplete widget.
+   */
+  function testTaxonomyTermFieldInstanceSettingsAutocompleteWidget() {
+    entity_get_form_display('entity_test', 'entity_test', 'default')
+      ->setComponent($this->field_name, array(
+        'type' => 'taxonomy_autocomplete',
+        'weight' => 1,
+      ))
+      ->save();
+    $this->drupalGet('entity_test/structure/entity_test/fields/entity_test.entity_test.' . $this->field_name);
+    $this->assertNoErrorsLogged();
+  }
+
   /**
    * Tests that vocabulary machine name changes are mirrored in field definitions.
    */