[scuter]

попробуй сделать так,

createWindow : function(){
        var desktop = this.app.getDesktop();
        var win = desktop.getWindow('FileManager-win');
        if(!win){
            win = desktop.createWindow({
                id: 'FileManager-win',
                title:'Мой компьютер',
                width:800,
                height:500,
                iconCls: 'tabs',
                shim:false,
                animCollapse:false,
                border:false,
                constrainHeader:true,
                layout: 'fit',
                items: FileManager
            });
            this.FileManagerWin = win;
        }
        this.FileManagerWin.show();
    }