Could not cast value of type 'NSSplitViewController' (0x7fff8fbe20c8) to 'AppName.ViewController' (0x100049988)

Question

I created a cocoa web browser. I added a NSSearchfield in my toolbar. I was testing it out then I got and Error saying Thread 1: Signal SIGART. In my debugger, it said

Could not cast value of type 'NSSplitViewController' (0x7fff8fbe20c8) to 'AppName.ViewController' (0x100049988).

This is happening because I linked the window controller to a split view controller instead of the viewcontroller. This is the code I got an error in

  var viewController: ViewController {
            get {
                return self.window!.contentViewController! as! ViewController
                // Error here
            }
        }

I have tried doing this

var SplitView: SplitViewContorller {
            get {
                return self.window!.contentViewController! as! SplitVC
                // Error here
            }
        }

But then I get numerous errors saying

Value of type 'SplitVC' has no member 'webView'

In this code

 var urlStr = searchFieldToolBar.stringValue
        if urlStr.hasPrefix("https://") {
            let url = NSURL(string: urlStr)
            let req = NSURLRequest(url:url! as URL)
            self.viewController.webView.load(req as URLRequest)
        }

I don't know how to fix this.

Can you please help me