I could reproduce the issue and have registered this as a bug.
Here is how you can work around this: Edit the form markup and remove the __xmlns:f="http://www.composite.net/ns/function/1.0"__ part inside <KeySelector.Options />. The namespace is already declared at the root element, so it can be removed without issue.
Sorry about the inconvenient and thanks for reporting this! We will get this fixed within a few days.
We changed how form markup is passed recently so this looks like a 4.0 beta issue only - are you on 4.0 also?
Here is how you can work around this: Edit the form markup and remove the __xmlns:f="http://www.composite.net/ns/function/1.0"__ part inside <KeySelector.Options />. The namespace is already declared at the root element, so it can be removed without issue.
Sorry about the inconvenient and thanks for reporting this! We will get this fixed within a few days.
We changed how form markup is passed recently so this looks like a 4.0 beta issue only - are you on 4.0 also?