React: d3.event is null in a ReactJS + d3JS component

I'm using ReactJS, d3JS and ES6 to create an org chart. I can create the chart and see it. But I want to add the zoom and drag behavior to the chart, so I used d3.behavior.zoom. I can see the method zoomed is called, but the d3.event is null.

I tried attaching the zoom behavior to the div, svg and g but nothing helps. The zoomed behavior is called but the event is null.

In my ASP.NET project I made sure that I don't have any reference to d3.js other than in the systemJS configuration which many stackoverflow answers mentioned as the issue related to why d3.event is null.

My code is very similar to what is in an example like this https://github.com/Swizec/candidate-bucket-chart/blob/169779e110c8825f4545c71ae5845ff7bad4f1f4/src/BubbleChart.jsx which uses the old version of ReactJS.

This is my systemJS configuration:

<script>
    System.defaultJSExtensions = true;
    System.config({
        map: {
            'rx': "https://cdnjs.cloudflare.com/ajax/libs/rxjs/4.0.7/rx.all.min.js",
            'react': 'https://cdnjs.cloudflare.com/ajax/libs/react/15.0.1/react.js',
            'react-dom': 'https://cdnjs.cloudflare.com/ajax/libs/react/15.0.1/react-dom.js',
            'd3': 'https://cdnjs.cloudflare.com/ajax/libs/d3/3.5.16/d3.js'
        }
    });
    System.import('../app/app.js');
</script>

import React, { Component } from 'react';
import * as Rx from 'rx';
import * as d3 from 'd3';
import Person from './person';

class OrgChart extends Component {

    constructor(props) {
        super(props);

        this.state = { persons: [] };
    }

    componentWillMount() {
        // Just read a JSON file
        var source = Rx.Observable.fromPromise($.getJSON("/js/org-persons.json"));

        source.subscribe(
            function(chart) {
                var tree = d3.layout.tree()
                    .nodeSize([110, 50])
                    .separation(function() {
                        return 1;
                    });

                var nodes = tree.nodes(chart[0]).reverse();

                var links = tree.links(nodes);

                var p = [];

                nodes.forEach(function(node) {
                    fs.push((<Person x={node.x} y={node.y}></Person>));
                }.bind(this));

                this.setState({ person: p });

            }.bind(this)
        );
    }

    componentDidMount() {
        var rootX = document.body.clientWidth / 4;
        var rootY = 300;

       // A standard setup with d3 to use it's zoom/drag behavior
        var zoom = d3.behavior.zoom()
            .scaleExtent([1, 10])
            .on("zoom", this.zoomed)
            .translate([rootX, rootY]);

       // Attach zoom behavior to the first group in the svg.
        d3.select("#viewport")
            .call(zoom);
    }

    // When I drag the svg, this function gets called but the event is null. d3 is a global object.
    zoomed() {
        d3.select("#viewport").attr("transform", "translate(" + d3.event.translate + ")scale(" + d3.event.scale + ")");
    }

   // renders an org chart
    render() {

        return (
            <div id='treeContainer'><svg id='fullTree' width='100%'>
                <g id='viewport'>
                    { this.state.persons }
                </g>
            </svg></div>
        );
    }
}

export default OrgChart;