Working around "IndexError: tuple index out of range" when using Solgema.fullcalendar to add events

I'm in no way sure if this is ok since the problem only happend when adding an p.a.event-dx in Solgema.fullcalendar by clicking on the calendar (which sets the widget-values programmatically - maybe wrongly). 

But: the tests pass :-)

Here is the traceback:

```
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 Solgema.fullcalendar.browser.dx, line 64, in __call__
  Module plone.z3cform.layout, line 60, in update
  Module z3c.form.form, line 263, in render
  Module z3c.form.form, line 159, in render
  Module Products.Five.browser.pagetemplatefile, line 125, in __call__
  Module Products.Five.browser.pagetemplatefile, line 59, in __call__
  Module zope.pagetemplate.pagetemplate, line 132, in pt_render
  Module five.pt.engine, line 93, in __call__
  Module z3c.pt.pagetemplate, line 149, in render
  Module chameleon.zpt.template, line 257, in render
  Module chameleon.template, line 172, in render
  Module c673b4c135a1f2f8ec6bac730659b250834585ca.py, line 256, in render
  Module 055c1fc0e8c83139eb526be940a6ebfca8ec918c.py, line 1477, in render_master
  Module 055c1fc0e8c83139eb526be940a6ebfca8ec918c.py, line 687, in render_content
  Module c673b4c135a1f2f8ec6bac730659b250834585ca.py, line 241, in __fill_main
  Module 5976bb142c7f975d3b500549123c9fa2e3fca6ad.py, line 1729, in render_titlelessform
  Module 5976bb142c7f975d3b500549123c9fa2e3fca6ad.py, line 451, in render_fields
  Module 5976bb142c7f975d3b500549123c9fa2e3fca6ad.py, line 126, in render_widget_rendering
  Module 5976bb142c7f975d3b500549123c9fa2e3fca6ad.py, line 1041, in render_field
  Module five.pt.expressions, line 161, in __call__
  Module Products.Five.browser.metaconfigure, line 476, in __call__
  Module zope.browserpage.viewpagetemplatefile, line 83, in __call__
  Module zope.browserpage.viewpagetemplatefile, line 51, in __call__
  Module zope.pagetemplate.pagetemplate, line 132, in pt_render
  Module five.pt.engine, line 93, in __call__
  Module z3c.pt.pagetemplate, line 149, in render
  Module chameleon.zpt.template, line 257, in render
  Module chameleon.template, line 172, in render
  Module 717f95a505f594eed94f03dd6656d28f1bd70abe.py, line 555, in render
  Module 717f95a505f594eed94f03dd6656d28f1bd70abe.py, line 438, in render_widget_wrapper
  Module five.pt.expressions, line 161, in __call__
  Module z3c.form.widget, line 153, in render
  Module zope.browserpage.viewpagetemplatefile, line 51, in __call__
  Module zope.pagetemplate.pagetemplate, line 132, in pt_render
  Module five.pt.engine, line 93, in __call__
  Module z3c.pt.pagetemplate, line 149, in render
  Module chameleon.zpt.template, line 257, in render
  Module chameleon.template, line 190, in render
  Module chameleon.template, line 172, in render
  Module c306234bfd2eeb7d1c3c99330648ee7bb59fe1b4.py, line 2292, in render
  Module five.pt.expressions, line 154, in __call__
  Module five.pt.expressions, line 126, in traverse
  Module zope.traversing.adapters, line 136, in traversePathElement
   - __traceback_info__: (<DatetimeWidget 'form.widgets.IEventBasic.start'>, 'timezone')
  Module zope.traversing.adapters, line 42, in traverse
   - __traceback_info__: (<DatetimeWidget 'form.widgets.IEventBasic.start'>, 'timezone', ())
  Module plone.formwidget.datetime.base, line 449, in timezone
IndexError: tuple index out of range

 - Expression: "widget/@@ploneform-render-widget"
 - Filename:   ... rm-0.7.2-py2.7.egg/plone/app/z3cform/templates/macros.pt
 - Location:   (94:81)
 - Source:     ... place="structure widget/@@ploneform-render-widget"/>
                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 - Expression: "view/timezone"
 - Filename:   ... /formwidget/datetime/z3cform/templates/datetime_input.pt
 - Location:   (182:60)
 - Source:     ... view/name}-timezone; value view/timezone"/>
                                              ^^^^^^^^^^^^^
 - Arguments:  repeat: {...} (0)
               context: <ImplicitAcquisitionWrapper kalender at 0x10dd82230>
               views: <ViewMapper - at 0x10d0416d0>
               modules: <TraversableModuleImporter - at 0x107de7b50>
               args: <tuple - at 0x106d82050>
               nothing: <NoneType - at 0x106d11148>
               target_language: <NoneType - at 0x106d11148>
               default: <object - at 0x106dc6b70>
               request: <instance - at 0x10e03bc20>
               wrapped_repeat: {...} (1)
               loop: {...} (1)
               template: <ViewPageTemplateFile - at 0x10b6b9510>
               translate: <function translate at 0x10d04a758>
               options: {...} (0)
               view: <DatetimeWidget IEventBasic.start at 0x10d05ce10>
```