Accept "add parameter" refactoring button disappears when editing arg name

In GT 0.8.950, when refactoring to add a parameter and then editing the default argument name that is created, the button to accept the refactoring disappears when the last character of the default argument name is deleted.

This relates to all in place refactorings.

The button is added as an attribute. When you are deleting the last character, the button goes away as well.

The refactoring cancels now when editing outside of it.