React this.state is undefined? : <p>I am following a beginner tutorial from Pluralsight, on form submit a value is passed to <code>addUser</code> component method and I need to push userName to <code>this.state.users</code> but I get error </p>

<pre><code> App.jsx:14 Uncaught TypeError: Cannot read property 'users' of undefined
</code></pre>

<p>Component</p>

<pre><code>import React from 'react'
import User from 'user'
import Form from 'form'

class Component extends React.Component {
    constructor() {
        super()
        this.state = {
            users: null
        }
    }
    // This is triggered on form submit in different component
    addUser(userName) { 
        console.log(userName) // correctly gives String
        console.log(this.state) // this is undefined
        console.log(this.state.users) // this is the error
        // and so this code doesn't work
        /*this.setState({
            users: this.state.users.concat(userName)
        })*/
    }
    render() {
        return (
            &lt;div&gt;
            &lt;Form addUser={this.addUser}/&gt;
            &lt;/div&gt;
            )
    }
}

export default Component
</code></pre>
