Get() on array in settings from custom Form controller
You can handle the config like an array:

$form['infoblock_text'] = [
    '#type'          =&gt; 'text_format',
    '#title'         =&gt; t('Infoblock text'),
    '#description'   =&gt; t('The body text for the infoblock on the front page'),
    '#format'        =&gt; isset($config-&gt;get('infoblock_text')['format']) ? $config-&gt;get('infoblock_text')['format'] : 'full_html',
    '#default_value' =&gt; isset($config-&gt;get('infoblock_text')['value']) ? $config-&gt;get('infoblock_text')['value'] : '',
];


When accessing the array like this, it's always a good idea to check if the key is set. This avoids warnings in your log.