Little problem

The app is great, it really helps people to learn swift quickly and easily with lots of fun) but I have a problem(( passing the chapter "string" at 16 step I reached a problem... When tap "go on" button the app stops and I cant go on. Its very pity(( pleas fix this problem. Thank you a lot!)

Alexdmit999 about Swifty: Learn to code tutorials for Swift, v1.7