diff --git a/core/lib/Drupal.php b/core/lib/Drupal.php
index 7569746..9efc0a1 100644
--- a/core/lib/Drupal.php
+++ b/core/lib/Drupal.php
@@ -125,15 +125,9 @@ class Drupal {
    * Sets a new global container.
    *
    * @param \Symfony\Component\DependencyInjection\ContainerInterface $container
-   *   A new container instance to replace the current. NULL may be passed by
-   *   testing frameworks to ensure that the global state of a previous
-   *   environment does not leak into a test.
+   *   A new container instance to replace the current.
    */
-  public static function setContainer(ContainerInterface $container = NULL) {
-    if (!isset($container)) {
-      // @todo Remove the NULL case, and use unsetContainer() instead.
-      $container = new PlaceholderContainer('\Drupal::$container was unset with setContainer(NULL).');
-    }
+  public static function setContainer(ContainerInterface $container) {
     static::$container = $container;
   }
 
diff --git a/core/modules/simpletest/src/TestBase.php b/core/modules/simpletest/src/TestBase.php
index b4901af..a6d1bf2 100644
--- a/core/modules/simpletest/src/TestBase.php
+++ b/core/modules/simpletest/src/TestBase.php
@@ -1187,7 +1187,7 @@ private function prepareEnvironment() {
 
     // Ensure there is no service container.
     $this->container = NULL;
-    \Drupal::setContainer(NULL);
+    \Drupal::unsetContainer();
 
     // Unset globals.
     unset($GLOBALS['config_directories']);
diff --git a/core/modules/system/src/Tests/Bootstrap/GetFilenameUnitTest.php b/core/modules/system/src/Tests/Bootstrap/GetFilenameUnitTest.php
index a7bcca7..1636b1f 100644
--- a/core/modules/system/src/Tests/Bootstrap/GetFilenameUnitTest.php
+++ b/core/modules/system/src/Tests/Bootstrap/GetFilenameUnitTest.php
@@ -16,10 +16,32 @@
  */
 class GetFilenameUnitTest extends KernelTestBase {
 
+  /**
+   * The container used by the test, moved out of the way.
+   *
+   * @var \Symfony\Component\DependencyInjection\ContainerInterface
+   */
+  protected $previousContainer;
+
+  /**
+   * {@inheritdoc}
+   */
   protected function setUp() {
     parent::setUp();
+    // Store the previous container.
+    $this->previousContainer = $this->container;
     $this->container = NULL;
-    \Drupal::setContainer(NULL);
+    \Drupal::unsetContainer();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function tearDown() {
+    parent::tearDown();
+    // Restore the previous container.
+    $this->container = $this->previousContainer;
+    \Drupal::setContainer($this->previousContainer);
   }
 
   /**
diff --git a/core/modules/system/src/Tests/Routing/RouteProviderTest.php b/core/modules/system/src/Tests/Routing/RouteProviderTest.php
index be8862c..6ba9f97 100644
--- a/core/modules/system/src/Tests/Routing/RouteProviderTest.php
+++ b/core/modules/system/src/Tests/Routing/RouteProviderTest.php
@@ -50,6 +50,7 @@ class RouteProviderTest extends KernelTestBase {
   protected $state;
 
   protected function setUp() {
+    parent::setUp();
     $this->fixtures = new RoutingFixtures();
     $this->routeBuilder = new NullRouteBuilder();
     $this->state = new State(new KeyValueMemoryFactory());
diff --git a/core/tests/Drupal/Tests/UnitTestCase.php b/core/tests/Drupal/Tests/UnitTestCase.php
index d6deb9e..e82c3a9 100644
--- a/core/tests/Drupal/Tests/UnitTestCase.php
+++ b/core/tests/Drupal/Tests/UnitTestCase.php
@@ -39,7 +39,7 @@ protected function setUp() {
     parent::setUp();
     // Ensure that an instantiated container in the global state of \Drupal from
     // a previous test does not leak into this test.
-    \Drupal::setContainer(NULL);
+    \Drupal::unsetContainer();
 
     $this->root = dirname(dirname(substr(__DIR__, 0, -strlen(__NAMESPACE__))));
   }