diff --git a/core/lib/Drupal/Component/Datetime/Time.php b/core/lib/Drupal/Component/Datetime/Time.php
index ec660094fc9..729c762f12a 100644
--- a/core/lib/Drupal/Component/Datetime/Time.php
+++ b/core/lib/Drupal/Component/Datetime/Time.php
@@ -31,7 +31,7 @@ class Time implements TimeInterface {
    * @param \Symfony\Component\HttpFoundation\RequestStack|null $request_stack
    *   (Optional) The request stack.
    */
-  public function __construct(RequestStack $request_stack = NULL) {
+  public function __construct(?RequestStack $request_stack = NULL) {
     $this->requestStack = $request_stack;
   }
 
diff --git a/core/lib/Drupal/Component/DependencyInjection/Dumper/OptimizedPhpArrayDumper.php b/core/lib/Drupal/Component/DependencyInjection/Dumper/OptimizedPhpArrayDumper.php
index d4d794139e5..b2a271a284f 100644
--- a/core/lib/Drupal/Component/DependencyInjection/Dumper/OptimizedPhpArrayDumper.php
+++ b/core/lib/Drupal/Component/DependencyInjection/Dumper/OptimizedPhpArrayDumper.php
@@ -469,7 +469,7 @@ class OptimizedPhpArrayDumper extends Dumper {
    * @return string|object
    *   A suitable representation of the service reference.
    */
-  protected function getReferenceCall($id, Reference $reference = NULL) {
+  protected function getReferenceCall($id, ?Reference $reference = NULL) {
     $invalid_behavior = ContainerInterface::EXCEPTION_ON_INVALID_REFERENCE;
 
     if ($reference !== NULL) {
diff --git a/core/lib/Drupal/Component/Plugin/CategorizingPluginManagerInterface.php b/core/lib/Drupal/Component/Plugin/CategorizingPluginManagerInterface.php
index a7099fbdb5e..d6d1e37067e 100644
--- a/core/lib/Drupal/Component/Plugin/CategorizingPluginManagerInterface.php
+++ b/core/lib/Drupal/Component/Plugin/CategorizingPluginManagerInterface.php
@@ -25,7 +25,7 @@ interface CategorizingPluginManagerInterface extends PluginManagerInterface {
    * @return array[]
    *   An array of plugin definitions, sorted by category and label.
    */
-  public function getSortedDefinitions(array $definitions = NULL);
+  public function getSortedDefinitions(?array $definitions = NULL);
 
   /**
    * Gets sorted plugin definitions grouped by category.
@@ -41,6 +41,6 @@ interface CategorizingPluginManagerInterface extends PluginManagerInterface {
    *   Keys are category names, and values are arrays of which the keys are
    *   plugin IDs and the values are plugin definitions.
    */
-  public function getGroupedDefinitions(array $definitions = NULL);
+  public function getGroupedDefinitions(?array $definitions = NULL);
 
 }
diff --git a/core/lib/Drupal/Component/Plugin/Exception/InvalidPluginDefinitionException.php b/core/lib/Drupal/Component/Plugin/Exception/InvalidPluginDefinitionException.php
index 684f71afe97..10edb0265e0 100644
--- a/core/lib/Drupal/Component/Plugin/Exception/InvalidPluginDefinitionException.php
+++ b/core/lib/Drupal/Component/Plugin/Exception/InvalidPluginDefinitionException.php
@@ -28,7 +28,7 @@ class InvalidPluginDefinitionException extends PluginException {
    *
    * @see \Exception
    */
-  public function __construct($plugin_id, $message = '', $code = 0, \Exception $previous = NULL) {
+  public function __construct($plugin_id, $message = '', $code = 0, ?\Exception $previous = NULL) {
     $this->pluginId = $plugin_id;
     parent::__construct($message, $code, $previous);
   }
diff --git a/core/lib/Drupal/Component/Plugin/Exception/PluginNotFoundException.php b/core/lib/Drupal/Component/Plugin/Exception/PluginNotFoundException.php
index 7c0da9fe942..af87c4f8e21 100644
--- a/core/lib/Drupal/Component/Plugin/Exception/PluginNotFoundException.php
+++ b/core/lib/Drupal/Component/Plugin/Exception/PluginNotFoundException.php
@@ -21,7 +21,7 @@ class PluginNotFoundException extends PluginException {
    *
    * @see \Exception
    */
-  public function __construct($plugin_id, $message = '', $code = 0, \Exception $previous = NULL) {
+  public function __construct($plugin_id, $message = '', $code = 0, ?\Exception $previous = NULL) {
     if (empty($message)) {
       $message = sprintf("Plugin ID '%s' was not found.", $plugin_id);
     }
diff --git a/core/lib/Drupal/Component/Utility/EmailValidator.php b/core/lib/Drupal/Component/Utility/EmailValidator.php
index 9e6a0a06cc0..f1345d03b1f 100644
--- a/core/lib/Drupal/Component/Utility/EmailValidator.php
+++ b/core/lib/Drupal/Component/Utility/EmailValidator.php
@@ -23,7 +23,7 @@ class EmailValidator extends EmailValidatorUtility implements EmailValidatorInte
    * @return bool
    *   TRUE if the address is valid.
    */
-  public function isValid($email, EmailValidation $email_validation = NULL) {
+  public function isValid($email, ?EmailValidation $email_validation = NULL) {
     if ($email_validation) {
       throw new \BadMethodCallException('Calling \Drupal\Component\Utility\EmailValidator::isValid() with the second argument is not supported. See https://www.drupal.org/node/2997196');
     }
diff --git a/core/lib/Drupal/Component/Utility/NestedArray.php b/core/lib/Drupal/Component/Utility/NestedArray.php
index 354d5818320..6d6d8b9ef58 100644
--- a/core/lib/Drupal/Component/Utility/NestedArray.php
+++ b/core/lib/Drupal/Component/Utility/NestedArray.php
@@ -358,7 +358,7 @@ class NestedArray {
    * @return array
    *   The filtered array.
    */
-  public static function filter(array $array, callable $callable = NULL) {
+  public static function filter(array $array, ?callable $callable = NULL) {
     $array = is_callable($callable) ? array_filter($array, $callable) : array_filter($array);
     foreach ($array as &$element) {
       if (is_array($element)) {
diff --git a/core/lib/Drupal/Component/Utility/Xss.php b/core/lib/Drupal/Component/Utility/Xss.php
index 2c143cddf84..9f584bbf5aa 100644
--- a/core/lib/Drupal/Component/Utility/Xss.php
+++ b/core/lib/Drupal/Component/Utility/Xss.php
@@ -56,7 +56,7 @@ class Xss {
    *
    * @ingroup sanitization
    */
-  public static function filter($string, array $allowed_html_tags = NULL) {
+  public static function filter($string, ?array $allowed_html_tags = NULL) {
     if (is_null($allowed_html_tags)) {
       $allowed_html_tags = static::$htmlTags;
     }