Issue #1848040 by yched: Fixed Fatal error on 'Manage display' after creating new field.

core/modules/field_ui/lib/Drupal/field_ui/DisplayOverview.php

@@ -101,7 +101,18 @@ public function form(array $form, array &$form_state) {
     // Field rows.
     foreach ($instances as $name => $instance) {
       $field = field_info_field($instance['field_name']);
-      $display = $instance['display'][$this->view_mode];
+
+      if (isset($instance['display'][$this->view_mode])) {
+        $display = $instance['display'][$this->view_mode];
+      }
+      else {
+        $display = array(
+          'type' => 'hidden',
+          'label' => 'above',
+          'weight' => 0,
+        );
+      }
+
       $table[$name] = array(
         '#attributes' => array('class' => array('draggable', 'tabledrag-leaf')),
         '#row_type' => 'field',

core/modules/field_ui/lib/Drupal/field_ui/Tests/ManageDisplayTest.php

@@ -167,6 +167,27 @@ function testViewModeCustom() {
     $this->assertNodeViewNoText($node, 'rss', $value, "The previous settings are kept when 'rss' mode is specialized again.");
   }
 
+  /**
+   * Tests that field instances with no explicit display settings do not break.
+   */
+  function testNonInitializedFields() {
+    // Create a test field.
+    $edit = array(
+      'fields[_add_new_field][label]' => 'Test',
+      'fields[_add_new_field][field_name]' => 'test',
+    );
+    $this->fieldUIAddNewField('admin/structure/types/manage/' . $this->type, $edit);
+
+    // Check that no settings have been set for the 'teaser' mode.
+    $instance = field_info_instance('node', 'field_test', $this->type);
+    $this->assertFalse(isset($instance['display']['teaser']));
+
+    // Check that the field appears as 'hidden' on the 'Manage display' page
+    // for the 'teaser' mode.
+    $this->drupalGet('admin/structure/types/manage/' . $this->type . '/display/teaser');
+    $this->assertFieldByName('fields[field_test][type]', 'hidden', 'The field is displayed as \'hidden \'.');
+  }
+
   /**
    * Tests hiding the view modes fieldset when there's only one available.
    */