Fixes #3121: `yii\base\Application::bootstrap` may fail to load some components…

framework/CHANGELOG.md

@@ -7,6 +7,7 @@ Yii Framework 2 Change Log
 - Bug #3042: `yii\widgets\Pjax` should end application right after it finishes responding to a pjax request (qiangxue)
 - Bug #3066: `yii\db\mssql\Schema::getTableSchema()` should return null when the table does not exist (qiangxue)
 - Bug #3091: Fixed inconsistent treatment of `Widget::run()` when a widget is used as a container and as a self-contained object (qiangxue)
+- Bug #3121: `yii\base\Application::bootstrap` may fail to load some components if they are specified as class names (qiangxue)
 - Bug #3128: Fixed the bug that `defaultRoles` set in RBAC manager was not working as specified (qiangxue)
 - Enh #3103: debugger panel is now not displayed when printing a page (githubjeka)
 - Enh #3108: Added `yii\debug\Module::enableDebugLogs` to disable logging debug logs by default (qiangxue)

framework/base/Application.php

@@ -276,6 +276,7 @@ abstract class Application extends Module
         }
 
         foreach ($this->bootstrap as $class) {
+            $component = null;
             if (is_string($class)) {
                 if ($this->has($class)) {
                     $component = $this->get($class);