Possible Unhandled Promise Rejection (id: 0): TypeError: TypeError: Cannot read property 'email' of null

I have problem with my react native projects. When i Log in with firebase and i will store data to AsyncStorage it always get an error message when first time Log in. But at the second time this error not show. I'm really confusing about this problem.

Dashboard.js

constructor(props){
    super(props);
    this.state= {
        user: null,
        loading: true,
    }
}

componentWillMount() {
    AsyncStorage.getItem('userData').then((user_data_json) => {
        let userData = JSON.parse(user_data_json);
        this.setState({
            user: userData,
            loading:false
        });
        console.log(this.state.user)
    });
}

Logout(){
    AsyncStorage.removeItem('userData').then(() => {
        firebase.auth().signOut().then(() => {
            this.props.navigation.navigate('Login')
        });

    });
    console.log(this.state.user)
}

render() {
    if(this.state.loading){ 
        return <ActivityIndicator size="large" />
    }
    return(
        <View>
            <Text style={styles.title}> Hello {this.state.user.email} </Text>
            <Button title="Log Out" onPress={this.Logout.bind(this)} />
        </View>
    )
}

}