Created an Active Choices Reactive Parameter with radio-buttons output. The Groovy code always marks the first radio-button as selected by adding ":selected" in the result.

The problem is that If the user does not change the (automatic) selection the AC Reactive Parameter has no value. If the user changes the (automatic) selection the parameter provides the user selected value. In case the AC Reactive Parameter generates a single choice the user will not be able to enforce a change, therefore the parameter will never have a value.

Here is an excerpt from the code (see the attached file for the full project):

if ( validStr != null ) {
  ...
  result[0] += ":selected"; //Select the first choice by default
}

return result;