diff --git a/core/lib/Drupal/Core/Field/FieldTypePluginManager.php b/core/lib/Drupal/Core/Field/FieldTypePluginManager.php
index 565fa4e6f7..40817d28b5 100644
--- a/core/lib/Drupal/Core/Field/FieldTypePluginManager.php
+++ b/core/lib/Drupal/Core/Field/FieldTypePluginManager.php
@@ -168,7 +168,7 @@ public function getPreconfiguredOptions($field_type) {
    */
   public function getPluginClass($type) {
     $plugin_definition = $this->getDefinition($type, FALSE);
-    return $plugin_definition['class'];
+    return $plugin_definition['class'] ?? NULL;
   }
 
 }
diff --git a/core/lib/Drupal/Core/Field/WidgetBase.php b/core/lib/Drupal/Core/Field/WidgetBase.php
index c3b67ba0b7..689b7128bc 100644
--- a/core/lib/Drupal/Core/Field/WidgetBase.php
+++ b/core/lib/Drupal/Core/Field/WidgetBase.php
@@ -456,8 +456,8 @@ public function flagErrors(FieldItemListInterface $items, ConstraintViolationLis
           }
           // Otherwise, pass errors by delta to the corresponding sub-element.
           else {
-            $original_delta = $field_state['original_deltas'][$delta];
-            $delta_element = $element[$original_delta];
+            $original_delta = $field_state['original_deltas'][$delta] ?? NULL;
+            $delta_element = $element[$original_delta] ?? NULL;
           }
           foreach ($delta_violations as $violation) {
             // @todo: Pass $violation->arrayPropertyPath as property path.
diff --git a/core/modules/hal/src/Normalizer/EntityReferenceItemNormalizer.php b/core/modules/hal/src/Normalizer/EntityReferenceItemNormalizer.php
index 48dbb81b47..55d641a823 100644
--- a/core/modules/hal/src/Normalizer/EntityReferenceItemNormalizer.php
+++ b/core/modules/hal/src/Normalizer/EntityReferenceItemNormalizer.php
@@ -82,7 +82,7 @@ public function normalize($field_item, $format = NULL, array $context = []) {
 
     // Normalize the target entity.
     $embedded = $this->serializer->normalize($target_entity, $format, $context);
-    $link = $embedded['_links']['self'];
+    $link = $embedded['_links']['self'] ?? NULL;
     // If the field is translatable, add the langcode to the link relation
     // object. This does not indicate the language of the target entity.
     if ($langcode) {
diff --git a/core/modules/link/src/Plugin/Field/FieldType/LinkItem.php b/core/modules/link/src/Plugin/Field/FieldType/LinkItem.php
index 373c3f03ad..f07c173336 100644
--- a/core/modules/link/src/Plugin/Field/FieldType/LinkItem.php
+++ b/core/modules/link/src/Plugin/Field/FieldType/LinkItem.php
@@ -188,7 +188,7 @@ public function setValue($values, $notify = TRUE) {
     }
     // Unserialize the values, this is deprecated as the storage takes care of
     // this, options must not be passed as a string anymore.
-    if (is_string($values['options'])) {
+    if (isset($values['options']) && is_string($values['options'])) {
       @trigger_error('Support for passing options as a serialized string is deprecated in 8.7.0 and will be removed before Drupal 9.0.0. Pass them as an array instead. See https://www.drupal.org/node/2961643.', E_USER_DEPRECATED);
       $values['options'] = unserialize($values['options'], ['allowed_classes' => FALSE]);
     }
diff --git a/core/modules/search/src/SearchQuery.php b/core/modules/search/src/SearchQuery.php
index 9e47cf1a42..6e342ca202 100644
--- a/core/modules/search/src/SearchQuery.php
+++ b/core/modules/search/src/SearchQuery.php
@@ -246,7 +246,7 @@ protected function parseSearchExpression() {
 
       // Strip off phrase quotes.
       $phrase = FALSE;
-      if ($match[2]{0} == '"') {
+      if ($match[2][0] == '"') {
         $match[2] = substr($match[2], 1, -1);
         $phrase = TRUE;
         $this->simple = FALSE;
diff --git a/core/tests/Drupal/KernelTests/KernelTestBase.php b/core/tests/Drupal/KernelTests/KernelTestBase.php
index dfda4a9fc0..290e494aea 100644
--- a/core/tests/Drupal/KernelTests/KernelTestBase.php
+++ b/core/tests/Drupal/KernelTests/KernelTestBase.php
@@ -605,7 +605,7 @@ protected function tearDown() {
 
     // Remove all prefixed tables.
     $original_connection_info = Database::getConnectionInfo('simpletest_original_default');
-    $original_prefix = $original_connection_info['default']['prefix']['default'];
+    $original_prefix = $original_connection_info['default']['prefix']['default'] ?? NULL;
     $test_connection_info = Database::getConnectionInfo('default');
     $test_prefix = $test_connection_info['default']['prefix']['default'];
     if ($original_prefix != $test_prefix) {