Think for yourself:
dijit.byId('_fromState_id').set('disabled', true);
just doing the job. Change it to falseto enable the widget.
Greetings.
UPDATE
In addition, there is another attribute of the widget called "readOnly", the difference from it "disabled"is as follows:
:
dijit.byId("_fromState_id").set("value", "NOTAVAILABLE");
dijit.byId('_fromState_id').set('readOnly', true);