var layoutView = new Layout();
var itemView = new ItemView();

App.containerRegion.show(layoutView);
layoutView.mainRegion.show(itemView); <---- This is where the issue would occur.

render: function(){
    if (this.beforeRender){ this.beforeRender(); }
    this.trigger("before:render", this);
    this.trigger("item:before:render", this);

    var data = this.serializeData();
    var templateSrc = this.getTemplate();
    $.ajax({
      url: 'templatesFolder/ + templateSrc,
      success: _bind(function(template){
         var html = Marionette.Renderer.render(template, data);
         this.$el.html(html);

         if (this.onRender){ this.onRender(); }
         this.trigger("render", this);
         this.trigger("item:rendered", this);
      }, this)
    })

    return this;
  },