[Stopwatch] Add null check for id argument in Section::open()

UPGRADE-5.0.md

@@ -456,6 +456,11 @@ Serializer
  * The `AbstractNormalizer::handleCircularReference()` method has two new `$format` and `$context` arguments.
  * Removed support for instantiating a `DataUriNormalizer` with a default MIME type guesser when the `symfony/mime` component isn't installed.
 
+Stopwatch
+---------
+
+ * Removed support of passing `null` as 1st (`$id`) argument of `Section::get()` method, pass a valid child section identifier instead.
+
 Translation
 -----------
 

src/Symfony/Component/Stopwatch/Section.php

@@ -58,7 +58,7 @@ public function __construct(float $origin = null, bool $morePrecision = false)
      *
      * @return self|null The child section or null when none found
      */
-    public function get(?string $id)
+    public function get(string $id)
     {
         if (null === $id) {
             @trigger_error(sprintf('Passing "null" as the first argument of the "%s()" method is deprecated since Symfony 4.4, pass a valid child section identifier instead.', __METHOD__), E_USER_DEPRECATED);
@@ -80,7 +80,7 @@ public function get(?string $id)
      */
     public function open(?string $id)
     {
-        if (null === $session = $this->get($id)) {
+        if (null === $id || null === $session = $this->get($id)) {
             $session = $this->children[] = new self(microtime(true) * 1000, $this->morePrecision);
         }