Fabric.js - Change Rectangle Fill

Fixed the problem by using basic set method of fabric:

backgroundColor.onchange = function() {         
    canvas.getActiveObject().set("fill", this.value);
    canvas.renderAll();
};

tri this the easy and simple code by fabrics documentation

var activeObject = canvas.getActiveObject();
        activeObject.fill = 'your color code value';
        canvas.renderAll();