Fixed iOS crash due to force unwrap of nil

ios/Classes/QRView.swift

@@ -26,10 +26,8 @@ public class QRView:NSObject,FlutterPlatformView {
                 try scanner?.startScanning(resultBlock: { codes in
                     if let codes = codes {
                         for code in codes {
-                            if (code != nil && code.stringValue != nil) {
-                                let stringValue = code.stringValue!
-                                self.channel.invokeMethod("onRecognizeQR", arguments: stringValue)
-                            }
+                            guard let stringValue = code.stringValue else { continue }
+                            self.channel.invokeMethod("onRecognizeQR", arguments: stringValue)
                         }
                     }
                 })