ViewController.buildRenderer fatal error: unexpectedly found nil - line 321

[Optimus bits]

fatal error: unexpectedly found nil while unwrapping an Optional value
2017-09-13 21:58:03.487280+0200 Laufpark[41792:2840346] fatal error: unexpectedly found nil while unwrapping an Optional value

in ViewController.buildRenderer on line

let strokeColor: I<UIColor> = I(constant: lines[line]!.uiColor)