I love ExtJS, but some days it just does not love me back.
For instance, if you have a hidden textfield, and you don’t set the width property, it won’t be added to your form. Sure, you say, why would you need the width for a hidden input? Well, ExtJS thinks you might call myHiddenTextfield.show() some day, so it still needs to put it in there with a width. No width, no love.
If you are having this error, it will result in form.findField(myHiddenTextfield) returning null, and you getting an error if you attempt to set the non-existent field.
Note: ExtJS admonishes the use of hidden fields, saying that the more advanced controls for AJAX/JSONP and POST/GET mean you don’t need them, but in some cases, it is just nice to have all your eggs in one basket, and not across multiple objects and scopes.