What application is this in?
Moving this to the Service Management discussion forum.
Are you clicking on "Add" or "Select"?
Add would not be the correct button if you are 'selecting an incident'
You would use the 'Select' button. Add would try to create a new master incident, not use the one you had selected.
The error is being caused by the form that loads when you click on the 'Add' button. The form contains the expression seen in the error, which fails with the modal form. There is an existing problem for that specific issue, 52115.