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Ref: https://stackoverflow.com/questions/45593986/null-is-not-an-object-evaluating-appnavigator-router

1) First check the version of your react-native-router-flux. If the version is 3, wrap your <Scene> with <Scene key="root" >:

const RouterComponent = () => {
    return (
        <Router>
            <Scene key="root">
                <Scene key="login" component={LoginForm} title="Please Login" />
            </Scene>
        </Router>
    );
};

2) For version 4, replace <Scene key="root"> with <Stack key="root"> and don't forget importing import {Stack} from 'react-native-router-flux:

import { Router, Scene, Stack } from 'react-native-router-flux';
    const RouterComponent = () => {
    return (
        <Router>
            <Stack key="root">
                <Scene key="login" component={LoginForm} title="Please Login" />
            </Stack>
        </Router>
    );
};