Combobox does not show selected value

[jfmateos2]

I have a first combobox with a change event listener.
When the user selects a value in this first combobox, its change listener creates a second combobox (propiedad) and assigns it the following store:

propiedad.store=Ext.create('Ext.data.Store', {                                   
   fields: ['valor'],                                    
   data : [{'valor':'HOME'},{'valor':'WORK'}]
});
propiedad.displayField= 'valor';        
propiedad.valueField= 'valor';
propiedad.setEditable(false);

The second combobox, when dropped-down, shows the options HOME and WORK properly, but when I click on either of these options, the combobox drops-up but remains empty, although I know that the value has been assigned through a alert(propiedad.value) in the change listener of this second combobox.

¿Why the combobox does not show the selected value?

[mitchellsimoens]

You shouldn't change the valueField or displayField after instantiation.