Sorry by my English

First, it happen since 4.2 that introduce the utility class Ext.container.Monitor.

I think found the problem.

The error "this.monitor is null" , appear when submit the form and immediately in next instruction destroys the container form.

It happen because the submit action is asynchronous then if the container was destroy before the action submit call this.monitor, it appear null !!.

Solution: be sure not destroy container's form before the submit action end or defer it enough. (100 mil. sec. work for me) until not get this error.

work example :

var fp = Ext.create('Ext.form.Panel',{
                                 url: "/data/form",

                                 baseParams: {'param1': 'test'}
//if not defer get this.monitor null error
Ext.defer(fp.destroy, 100, fp);