Issue #3084078 by lauriii, alexpott, Jaesin: AdminNegotiator::determineActiveTheme() does not adhere to the interface

core/modules/user/src/Theme/AdminNegotiator.php

@@ -72,7 +72,7 @@ class AdminNegotiator implements ThemeNegotiatorInterface {
    * {@inheritdoc}
    */
   public function determineActiveTheme(RouteMatchInterface $route_match) {
-    return $this->configFactory->get('system.theme')->get('admin');
+    return $this->configFactory->get('system.theme')->get('admin') ?: NULL;
   }
 
 }

core/modules/user/tests/src/Unit/Theme/AdminNegotiatorTest.php

@@ -0,0 +1,44 @@
+<?php
+
+namespace Drupal\Tests\user\Unit\Theme;
+
+use Drupal\Core\Entity\EntityTypeManagerInterface;
+use Drupal\Core\Routing\AdminContext;
+use Drupal\Core\Routing\RouteMatch;
+use Drupal\Core\Session\AccountInterface;
+use Drupal\Tests\UnitTestCase;
+use Drupal\user\Theme\AdminNegotiator;
+
+/**
+ * Tests AdminNegotiator class.
+ *
+ * @group user
+ * @coversDefaultClass \Drupal\user\Theme\AdminNegotiator
+ */
+class AdminNegotiatorTest extends UnitTestCase {
+
+  /**
+   * @dataProvider getThemes
+   */
+  public function testDetermineActiveTheme($admin_theme, $expected) {
+    $user = $this->prophesize(AccountInterface::class);
+    $config_factory = $this->getConfigFactoryStub(['system.theme' => ['admin' => $admin_theme]]);
+    $entity_type_manager = $this->prophesize(EntityTypeManagerInterface::class);
+    $admin_context = $this->prophesize(AdminContext::class);
+    $negotiator = new AdminNegotiator($user->reveal(), $config_factory, $entity_type_manager->reveal(), $admin_context->reveal());
+    $route_match = $this->prophesize(RouteMatch::class);
+    $this->assertSame($expected, $negotiator->determineActiveTheme($route_match->reveal()));
+  }
+
+  /**
+   * Provides a list of theme names to test.
+   */
+  public function getThemes() {
+    return [
+      ['seven', 'seven'],
+      [NULL, NULL],
+      ['', NULL],
+    ];
+  }
+
+}