processCheckboxes: count(): Argument #1 ($value) must be of type Countable|array, null given

Please add to the issue summary your code implementation that triggers this bug.

Update: The issue is not resolved by adding

if (!isset($element['#default_value']) || $element['#default_value'] == 0) {
  $element['#default_value'] = [];
}

Because then the checkboxes are not show in the webform when you accept the defaults without changing anything. They do appear though when you manually change a default option. Then all default options are shown.

We can't debug custom code, but conceptually, I understand the checkboxes default value should be an array and not an empty string.

The attached patch might fix your issue, you could change #default_value when you extend the WebformCheckboxes plugin.