docs: def _add(self):
          """
              Add function logic, override to implement different logic
              returns add widget or None
          """
          is_valid_form = True
          get_filter_args(self._filters)
          exclude_cols = self._filters.get_relation_cols()
          form = self.add_form.refresh()
  
          if request.method == "POST":
              self._fill_form_exclude_cols(exclude_cols, form)
              if form.validate():
                  self.process_form(form, True)
                  item = self.datamodel.obj()
                  form.populate_obj(item)
  
                  try:
                      self.pre_add(item)
                  except Exception as e:
                      flash(str(e), "danger")
                  else:
                      if self.datamodel.add(item):
                          self.post_add(item)
                      flash(*self.datamodel.message)
                  finally:
                      return None
              else:
                  is_valid_form = False
          if is_valid_form:
              self.update_redirect()
          return self._get_add_widget(form=form, exclude_cols=exclude_cols)