下面是最终的修复，使用HangarRash和matt的答案

// queue to handle the getter and setter of _mostRecentLocationReceivedQueue
let _mostRecentLocationReceivedQueue : DispatchQueue = DispatchQueue(
    label: "org.hobrink.WayAndSee.mostRecentLocationReceivedQueue",
    qos: .default, attributes: .concurrent)

var _mostRecentLocationReceived : CLLocation? = nil
var mostRecentLocationReceived : CLLocation {
    set (newValue) {
        _mostRecentLocationReceivedQueue.async (flags: .barrier, execute: {
            _mostRecentLocationReceived = newValue
        })
    }
    
    get {
        return _mostRecentLocationReceivedQueue.sync {
            
            // check if someone already set the variable
            if (_mostRecentLocationReceived != nil) {
                
                // yes, so return it
                return _mostRecentLocationReceived!
                
            } else {
                
                // not set so far, provide a helper value
                
                // first atempt, the current location from location manager
                if WTS.unique.WTS_LocationManager != nil {
                    if WTS.unique.WTS_LocationManager!.location != nil {
                        return WTS.unique.WTS_LocationManager!.location!
                    }
                }
                
                // second atempt the last stored center of the map
                return CLLocation(latitude: WaysMapOnScreenLastCenter.latitude,
                                  longitude: WaysMapOnScreenLastCenter.longitude)
            }
        }
    }
}