Symptoms
User Task: Item Reference is grayed out and cannot be checked or unchecked when needed.
Diagnoses
The behavior to check and disable is right the logic behind it though needs tweaking to allow for this scenario where there is no Workflow Setting Item Ref setup.
Resolution
We provided a Appit 1.4 patch for this.
Possible workaround will be to add a Item Reference to the Workflow Settings section then you'll be able to chose a Item Ref on the Task Event and move forward with the wizard.