What needs to be fixed

Let's say you want to display a date picker (calendar) in Joomla 4.0.0.b1. As per usual you do:

echo \Joomla\CMS\HTML\HTMLHelper::('calendar', $myDate, 'fieldName', 'fieldID');

You'd expect to see a Calendar field. However, if error reporting is set to Maximum you are met with a nasty PHP Notice "Notice: Undefined variable: dataAttribute in ...../layouts/joomla/form/field/calendar.php on line 117" This breaks the display and the calendar field.

Why this should be fixed

Because it breaks all of my software where a date needs to be selected and I don't wanna have to implement my own date picker.

How would you fix it

Change this https://github.com/joomla/joomla-cms/blob/4.0-dev/layouts/joomla/form/field/calendar.php#L117

to that:

<?php echo $dataAttribute ?? ''; ?>

Side Effects expected

None.