Concentrating to cast defensively states that if the check succeeds, the spell goes off, and if it fails, it fizzles, which isn't how we played it tonight. If it's intended to act as played tonight (i.e., the spell goes off, with or without the AoO), the text should be changed. That said, the rules as written right now make more sense. Otherwise it's free to try to concentrate, or at least at no greater risk than casting anyway. – bstern (talk) 04:39, 9 August 2019 (UTC)
You're right, I got the rule wrong. This isn't something I would want to change.