未定义不是本地对象反应

我得到了众所周知的错误未定义不是一个对象时,试图调用 本.refs 从导航选项的上下文。

最好我用代码解释一下:

 static navigationOptions = ({ navigation, screenProps }) => ({
        headerTitle: 'my app',
        title: 'my app',
        headerTitleStyle: {
            textAlign: "center",
            flex: 1,
        },
        headerLeft:
            <TouchableOpacity style={{padding: 15, marginLeft: 5}} onPress={() => { navigation.state.params.goBack(navigation.state.params.canGoBack) }}>
                {navigation.state.params.canGoBack ? <Text>back</Text>: <Text>close</Text>}
            </TouchableOpacity>,
    });


    componentDidMount() {
            this.props.navigation.setParams({goBack: this.onBack});
            this.props.navigation.setParams({canGoBack: this.state.canGoBack});
            Keyboard.dismiss();
        }

   onBack(canGoBack) {
        console.log(canGoBack);
        if(canGoBack){
            this.refs[WEBVIEW_REF].goBack(); // here is the error happening, somehow I can access this.refs.
        }
    }

    onNavigationStateChange(navState) {
        this.setState({
            canGoBack: navState.canGoBack
        });
        this.props.navigation.setParams({
            canGoBack: this.state.canGoBack,
        });
        console.log("some action happened: " + this.state.canGoBack);

    }

有人知道怎么解决这个问题吗?