Error creating a new user

[balkian]

Steps to reproduce:

• Open the create a new user form
• Fill the required fields
• Click on "register"

Expected result:

• A new user is added to the site, or an error message appears

Actual result:

• The page keeps "loading", and nothing happen
• If you check the console, this appears:

Traceback (innermost last):
  Module ZPublisher.Publish, line 126, in publish
  Module ZPublisher.mapply, line 77, in mapply
  Module Products.PDBDebugMode.runcall, line 70, in pdb_runcall
  Module ZPublisher.Publish, line 46, in call_object
  Module zope.formlib.form, line 795, in __call__
  Module five.formlib.formbase, line 50, in update
  Module zope.formlib.form, line 762, in update
  Module zope.formlib.form, line 703, in handleSubmit
  Module zope.formlib.form, line 616, in validate
  Module zope.formlib.form, line 552, in 
  Module plone.app.users.browser.register, line 261, in validate_registration
  Module zope.formlib.form, line 742, in validate
  Module zope.formlib.form, line 349, in getWidgetsData
  Module zope.formlib.widget, line 419, in getInputValue
  Module zope.schema._bootstrapfields, line 182, in validate
  Module zope.schema._field, line 380, in _validate
  Module zope.schema._bootstrapfields, line 211, in _validate
TypeError: unbound method study_fields() must be called with Validate instance a
s first argument (got str instance instead)