Steps to reproduce the issue

• Go to Content -> Fields
• Edit an existing field or create a new one
• Go to Options tab
• Edit the "Showon Attribute" field.
• Try to set it so that the field depends on another field being NOT EMPTY (without needing a specific value).

To show a field when another field (e.g. foo) has been selected (when that other field is not empty), use showon="foo!:" (without a value specified).

This is described in the documentation here:
https://docs.joomla.org/Form_field#Common_attributes
and here:
https://help.joomla.org/proxy?keyref=Help50:Fields:_Edit&lang=en#Options

Expected result

I should be able to save the field options.

Actual result

Upon saving, I get this error:

"Invalid field: Showon Attribute"

It seems like the form is expecting something after the colon, eg "foo!:2" (for saying when it's NOT 2), but as documented, the syntax for NOT EMPTY is simply "foo!:" and should work.

Now it no longer works.

System information (as much as possible)

‎5.0.0-beta1

Additional comments

This works correctly in the current and latest version Joomla 4.

It broke in Joomla 5.