I have a little problem with my code so that I can't call a function. I created a function `func changeLabelText(text: String) { mylabel.stringValue = text }` in the ViewController with that I can update the Text in the Label. When I try to call the function from another class in the Project, I get a runtime error(`EXC_BAD_Instr...`) and the Debugger holds on the line where I try to change the Label's text, with the error: `fatal error: unexpectedly found nil while unwrapping an Optional value`. Where is the problem? Can someone help me please!