Hack to avoid None.render

Caching the datagrid in the Column instance doesn't really work because
all instances of Datagrid share their column attributes.

Creating another local cache of the datatable I think will fix most of
our 'NoneType' object has no attribute 'render' errors, but there's
another race condition we're not catching where accessing

self.datatable.request

could be someone else's request!! (lines 207 and 276)