React Native Change Webview URL

Hi guys after a function call and its eventual callback I have to change the webview url but the error prints that this.webview is undefined, this is my code.
Into the function onMessage after I use postMessage from html I have to change the URL of the webview.
Help me and I will never stop thanking you

export default class WevViewApp extends Component {
constructor( props ) {
    super( props );
    this.webView = null;
}


onMessage( event ) {
   if(registerToken){
       //change url of my webview
    }
}

render() {
    return (
        <View style={{flex: 1}}>
            <WebView
                ref={( webView ) => this.webView = webView}
                style={{flex: 1}}
                source={{uri: 'https://url.com/test123'}}
                javaScriptEnabled={true}
                injectedJavaScript={myInjectedJs}
                onMessage={this.onMessage}
            />
        </View>
    );
}

}