diff --git a/core/lib/Drupal/Core/Extension/ThemeInstaller.php b/core/lib/Drupal/Core/Extension/ThemeInstaller.php
index d0ec674e43..1e06b4797d 100644
--- a/core/lib/Drupal/Core/Extension/ThemeInstaller.php
+++ b/core/lib/Drupal/Core/Extension/ThemeInstaller.php
@@ -106,19 +106,7 @@ public function __construct(ThemeHandlerInterface $theme_handler, ConfigFactoryI
     $this->routeBuilder = $route_builder;
     $this->logger = $logger;
     $this->state = $state;
-    $this->moduleInstaller = $module_installer;
-  }
-
-  /**
-   * Returns the module installer.
-   *
-   * @return \Drupal\Core\Extension\ModuleInstallerInterface
-   */
-  protected function getModuleInstaller() {
-    if (!isset($this->moduleInstaller)) {
-      $this->moduleInstaller = \Drupal::service('module_installer');
-    }
-    return $this->moduleInstaller;
+    $this->moduleInstaller = $module_installer ?: \Drupal::service('module_installer');
   }
 
   /**
@@ -149,7 +137,7 @@ public function install(array $theme_list, $install_dependencies = TRUE) {
         $theme_dependencies = array_diff_key($theme_data[$theme]->requires, $module_dependencies);
 
         // Install the module dependencies.
-        $this->getModuleInstaller()->install(array_keys($module_dependencies));
+        $this->moduleInstaller->install(array_keys($module_dependencies));
 
         // Add dependencies to the list of themes to install. The new themes
         // will be processed as the parent foreach loop continues.