title: Trying to access array offset on value of type null
body: <p>Migrating from php 7.1 to 7.4. We have like 500 functional tests for an API, and some of them started to fail with an error after the migration was complete. These tests were passing before everywhere, and now fail everywhere - not all, just 39.</p>
  
  <p>Environment information:</p>
  
  <ul>
  <li>php 7.4</li>
  <li>codeception</li>
  <li>yii2</li>
  </ul>
  
  <p>Stack trace:</p>
  
  <pre><code>...\api\vendor\codeception\codeception\src\Codeception\Subscriber\ErrorHandler.php:83
  ...\api\tests\functional\SomeFileHereCest.php:72
  ...\api\vendor\codeception\codeception\src\Codeception\Lib\Di.php:127
  ...\api\vendor\codeception\codeception\src\Codeception\Test\Cest.php:138
  ...\api\vendor\codeception\codeception\src\Codeception\Test\Cest.php:97
  ...\api\vendor\codeception\codeception\src\Codeception\Test\Cest.php:80
  ...\api\vendor\codeception\codeception\src\Codeception\Test\Test.php:88
  ... more stuff here, not important
  </code></pre>
  
  <p>Since <code>ErrorHandler.php:83</code> this is just catching the error, let's look at the <code>SomeFileHereCest.php:72</code>:</p>
  
  <pre><code>// declaration of the apiPrefix variable in the class.
  protected $apiPrefix;
  //...
  
  public function _before(FunctionalTester $I)
  {
      $this-&gt;apiPrefix = $this-&gt;config['backend']['api_prefix']; // this is the line 72
      //... more similar stuff later
  </code></pre>
  
  <p>So the <code>$this-&gt;config['backend']['api_prefix']</code> this is a string("v1")</p>
  
  <p>And I dont see where is the issue with this and how to dig into it deeper. Any ideas?</p>