FlutterError - Null check operator used on a null value - _ReferralPageState.initState.<fn>

[krushnarout]

_ReferralPageState.initState.

https://console.firebase.google.com/u/0/project/based-hardware/crashlytics/app/ios:com.friend-app-with-wearable.ios12/issues/c34e58418d1932973c1fdb1b4ccaf3d4?time=7d&types=crash&sessionEventKey=ebb1acf019264ee4947105909b486d35_2158476669594220838

logs:

          Fatal Exception: FlutterError
0  ???                            0x0 State.setState + 1219 (framework.dart:1219)
1  ???                            0x0 _ReferralPageState.initState.<fn> + 28 (referral_page.dart:28)
2  ???                            0x0 WebKitWebViewController._webView.<fn>.<fn> + 349 (webkit_webview_controller.dart:349)
3  ???                            0x0 NSObject.pigeon_setUpMessageHandlers.<fn> + 5050 (web_kit.g.dart:5050)
4  ???                            0x0 BasicMessageChannel.setMessageHandler.<fn> + 259 (platform_channel.dart:259)
5  ???                            0x0 _DefaultBinaryMessenger.setMessageHandler.<fn> + 653 (binding.dart:653)