Doesn't work on Firefox/IE, only on Chrome

[Vadorequest]

Hi.

i'm using an old version of this library, we've forked our own version based on react-dnd-html5-backend 2.1.2.

We never tried in another browser than Chrome until now and I get the following exception when trying to drag an Image:

TypeError: this.dragStartSourceIds is null

  HTML5Backend.prototype.handleDragStart = function handleDragStart(e, sourceId) {
    this.dragStartSourceIds.unshift(sourceId);
  };

It may be related to #29

Is there any fix for this? At least for FF?