Frankly, I do not understand your next steps well. Better start a new thread including all the code related and try to explain what you want to do and what you can do now.

If the line mapView.addOverlay(circles as! MKOverlay) causes Unexpectedly found nil, then mapView is nil when findSzczyty(_:) is called.

The crash log is suggesting that the crash happened while processing string interpolation, not when forced unwrapping. What sort of value are you passing to storeDomain?