When you add a resource and click cancel system crashes

When you add a resource, the add resource screen appears. If you do not write anything (does not add a title or a description) and clicks "cancel" the following error occurs:

Erro do Servidor
Um erro interno do servidor ocorreu. Isto provavelmente significa que você achou um bug no SchoolTool. Por favor, relate-o no rastreador de bugs.

Exceção
AttributeError: 'FlourishResourceAddView_addResource.html' object has no attribute '_resource'

Traceback
File "/usr/lib/python2.7/dist-packages/zope/publisher/publish.py", line 132, in publish
  result = publication.callObject(request, obj)
File "/usr/lib/python2.7/dist-packages/zope/app/publication/zopepublication.py", line 207, in callObject
  return mapply(ob, request.getPositionalArguments(), request)
File "/usr/lib/python2.7/dist-packages/zope/publisher/publish.py", line 107, in mapply
  return debug_call(obj, args)
Extra information: <schooltool.skin.flourish.zcml_content.FlourishResourceAddView_addResource.html object at 0xb563b90>
File "/usr/lib/python2.7/dist-packages/zope/publisher/publish.py", line 113, in debug_call
  return obj(*args)
File "/usr/lib/python2.7/dist-packages/schooltool/skin/flourish/page.py", line 67, in __call__
  self.update()
File "/usr/lib/python2.7/dist-packages/schooltool/skin/flourish/form.py", line 72, in update
  super(AddForm, self).update()
File "/usr/lib/python2.7/dist-packages/schooltool/skin/flourish/form.py", line 56, in update
  super(Form, self).update()
File "/usr/lib/python2.7/dist-packages/schooltool/resource/browser/resource.py", line 611, in update
  self.actions.execute()
File "/usr/lib/python2.7/dist-packages/z3c/form/action.py", line 99, in execute
  result = handler()
File "/usr/lib/python2.7/dist-packages/z3c/form/button.py", line 311, in __call__
  return handler(self.form, self.action)
File "/usr/lib/python2.7/dist-packages/z3c/form/button.py", line 170, in __call__
  return self.func(form, action)
File "/usr/lib/python2.7/dist-packages/schooltool/skin/flourish/form.py", line 83, in handleCancel
  self.request.response.redirect(self.nextURL())
File "/usr/lib/python2.7/dist-packages/schooltool/resource/browser/resource.py", line 674, in nextURL
  return absoluteURL(self._resource or self.context, self.request)