I got the following error in drupal:

Warning: Invalid argument supplied for foreach() in Drupal\Core\Render\Element\Checkboxes::valueCallback() (line 100 of core/lib/Drupal/Core/Render/Element/Checkboxes.php).

This is caused by a default value that is a string, and not an array.

Solution

 Make the default item an array of values:

            $form['item'] = array(
              '#type' => 'checkboxes',
              '#title' => 'Title'
              '#options' => [1,3],
              '#required' => TRUE,
              '#default_value' => [1]
            );