text_format elements are not supported

Problem/Motivation

When using text_format as child element it doesn't work correctly.

Steps to reproduce

Code example:

    $form['text'] = [
      '#type' => 'multivalue',
      '#title' => $this->t('Textbox'),
      '#required' => TRUE,
      'text' => [
        '#type' => 'text_format',
        '#title' => $this->t('Text'),
      ],
      '#default_value' => [
        0 => ['text' => ['value' => 'Text', 'format' => 'full_html']],
        1 => ['text' => ['value' => 'Another Text value', 'format' => 'full_html']],
      ],
    ];