extjs4 - ExtJS 4 - bbar collapsing after file upload thru 'file' widget -

i'm working extjs 4.1.1. i've 'filefield' config on bottom bar of panel. handle upload operation, i'm embedding widget dynamically created form. after form submit, bbar collapses. i've added alerts in listeners can called removal, , none of them invoked. behaviour perplexing. suggestions, please ? i've attached screen shots of bbar before , after upload.

bbar: [{             xtype: 'filefield',     id: 'fileuploadbtn',     buttononly: true,     buttontext: 'preview local file',     buttonconfig: {         iconcls: 'folder'     },     hidelabel: true,     listeners: {         change: function(field, value) {                         onuploadclick(field, docuploadurl, callback);         }      } }]  function onuploadclick(filefield, url, callback) {       var uploadform = ext.widget('form', {         autodestroy: false         ,items: [filefield]     });      uploadform.submit({         //...     });  }; 

filefield expects in form.
try inserting in form layout, , maybe won't happen:
http://docs.sencha.com/extjs/4.1.1/#!/api/ext.form.field.file