Swift, XCode - Annotation callout

so i am setting annotations on a map using a for-loop:

        for location in locations {
            let annotation = customAnnotation()
            annotation.title = location.title
            annotation.subtext= location.subtext
            annotation.coordinate = CLLocationCoordinate2D (latitude: location.latitude, longitude: location.longitude)
            map.addAnnotation(annotation)
        }
        self.map.delegate = self

and i am trying to print a subtext inside a callout, which is different for every annotation.

I am using this to describe the annotations properties:

    func mapView(_ mapView: MKMapView, viewFor annotation: MKAnnotation) -> MKAnnotationView? {
        guard let annot = annotation as? customAnnotation else { return nil }
        
        if annotation is MKUserLocation {
            return nil
        }
        let annotationView = MKAnnotationView(annotation: annotation, reuseIdentifier: "customAnnotation")
        annotationView.image = UIImage(named: annot.icon)
        annotationView.canShowCallout = true
        
        let rightButton = UIButton(type: .infoLight)
        rightButton.setImage(UIImage(named: annot.isCollapsed ? "ic_showmore" : "ic_showless"), for: .normal)
        annotationView.rightCalloutAccessoryView = rightButton
        
        if (annot.isCollapsed) {
            annotationView.detailCalloutAccessoryView = nil
        }
        else {
            print(annot.subtext)
            let label = UILabel(frame: CGRect(x: 0, y: 0, width: 200, height: 21))
            label.text = "text"
            label.font = UIFont.italicSystemFont(ofSize: 14.0)
            label.numberOfLines = 0
            annotationView.detailCalloutAccessoryView = label
            
            label.widthAnchor.constraint(lessThanOrEqualToConstant: label.frame.width).isActive = true
            label.heightAnchor.constraint(lessThanOrEqualToConstant: 90.0).isActive = true
        }
        return annotationView
    }

the if-else-part defines the annotations callout.

Now my problem is that the "label.text" is actually printed inside the callout, but only when its an actual text (example above: label.text = "text"). As soon as i try to fill in "annot.subtext", so there is a different text for every annotation, it shows a blank space.

When debugging i started putting print-statements in there (as seen in the code above just below the "else"). it printed my "annot.subtext" when i put the print-statement inside the if-part. But when i put "print(annot.subtext)" inside the else-part, it just showed me an error "CUICatalog invalid asset name supplied: ' ' ".

(the class i used to define my annotation:

class customAnnotation: MKPointAnnotation {
    var isCollapsed = true
    var setNeedsToggle = false
    
    var subtext: String = " "
    var icon: String = " "

Hope one of you can find a solution. Of course i am open for any questions.

Thanks a lot,

Gus