Check fancySelect.choicesInstance is set

After merging of joomla#46621, Cypress System Tests fail more often (but not always)
with `(uncaught exception)TypeError: Cannot read properties of undefined (reading 'clearChoices')`.
However, investigations have shown that this also happened before, but not as often.
It appears that Cypress speeds up page loading and XHR timing, and with the newly added `onChange()`, this causes Cypress to fail with an uncaught exception.
This simple *hack* prevents the crash. If you have a better implemention, you are welcome to share it.

Author: muhme

build/media_source/com_menus/js/admin-item-edit.es6.js

@@ -57,8 +57,16 @@ const onChange = ({ target }) => {
       const data = JSON.parse(response);
       const fancySelect = document.getElementById('jform_parent_id').closest('joomla-field-fancy-select');
 
-      fancySelect.choicesInstance.clearChoices();
-      fancySelect.choicesInstance.setChoices([{
+	const choices = fancySelect?.choicesInstance;
+
+	// If the field (or its Choices instance) isn't ready yet as in Cypress run, don't crash.
+	if (!choices) {
+	  return;
+	}
+
+	choices.clearChoices();
+	choices.setChoices([{
+
         id: '1',
         text: Joomla.Text._('JGLOBAL_ROOT_PARENT'),
       }], 'id', 'text', false);