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

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

Following the example "Adding a Ref to a Class Component" that is in this documentation I 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
            )                
    }
}

So I create ref in the constructor, associate it with the child component Display in the final function interpretJson() and try to use a method of child in my function of timeOut() , but I get the error :

"TypeError: Can not read property 'getCurrentIndex' of null "

What am I doing wrong not to be able to call the methods of Display to simulate the pseudo-code I have in comment?

EDIT : Here is 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 */ }