React-createRef() Api - this.child.Current is null

Question

In order to allow mine parent component access values of my child component (Display), I started working with the createRef() API that came out in this patch 16.3.

Following the example "Adding a Ref to a Class Component" which is in this documentation i made the following changes to my code:

class JsonFetcher extends Component {
    constructor(props) {
        super(props);
        this.child = React.createRef();
        this.state = {
            data: [],
        }
    }

    componentDidMount() {   
        this.updateContent(this.props.mainUrl)
    }

    updateContent(mainUrl){
        fetch(mainUrl)
            .then((responseJsonAnyUrl) => responseJsonAnyUrl.json())
            .then((responseJsonAnyUrl) => {
                this.setState({
                    mainUrl: mainUrl,
                    jsonObject: responseJsonAnyUrl
                },
                function () {
                    this.timeout = setTimeout(
                        function(){
                            //let ind = this.child.current.getCurrentIndex
                            //tyring to get values from child...
                            //if index === length-1
                            this.updateContent(mainUrl)
                            //else

                            //retry this function in 5 seconds
                            // 
                        }.bind(this, mainUrl)
                        ,
                        20000)
                }.bind(this))
            })
    }

    interpretJson() {
        /*
        *some code
        */            
        return(
            <Display content={contentListArray} ref={this.child}/>
        )
    }

    render() {
        if(this.state.jsonObject){
            return (
                <div>
                    <div> {this.interpretJson()} </div>
                </div>
            )
        }else
            return(
                null
            )                
    }
}

Therefore, I create the ref in the constructor, associate it with Child Component Display at the end function interpretJson() and try to use a method of child in my role of timeOut(), but I get the error:

"Typeerror: Cannot read Property 'getCurrentIndex' of null "

What am I doing wrong so I can’t call the methods of Display in order to simulate the pseudo-code I have in?

EDIT: Here is the Display

class Display extends Component{
    constructor(props){
        console.log('Display constructor was called')
        super(props);
        this.state = {
            contentIndex: -1
        }
    this.switchContent = this.switchContent.bind(this)
    }

    /*some methods */

    getCurrentIndex(){
        return this.state.contentIndex
    }

    getZoneLengths(){
        let zoneLengths = []
        this.props.content.map((zoneX, index) => {
            zoneLengths.push(zoneX.length)
        })        
        return zoneLengths

    }

     render(){ /* some code */ }