Another bug, which seems to pop up in different places:
Fatal error: Uncaught TypeError: Argument 2 passed to Form::hide() must be of the type string, null given, called in /xxxxxx/xxxxxx/xxxxxx/catalog/admin/customers.php on line 56 and defined in /xxxxxx/xxxxxx/xxxxxx/catalog/includes/system/versioned/1.0.8.1/form.php:73
The hide function forces its two parameters to be strings, which seems like a good idea. But then what happens, as in this case, when the second parameter is $customer_data->get('default_address_id', $customer_details) and that function/method returns null instead of a string?
As a workaround for now, I'll put "(string)" in front of that second parameter in my installation.