Interesting one, I'm surprised how I didn't bumped into this.

This was caused by the validation of the input which was done at the field level instead of being done when you click the "Generate Circuits" button not allowing you to enter a null / 0 number.

The validation was moved from the field to the button and now this should allow you to delete the numbers one by one and type the number of ways desired.

Also with this occasion, the "Generate Circuits" button was renamed "Generate Layout" as it describes the action better.

Thanks for reporting this.