docs: def on_add(self, widget, data=None):
          """Create a global variable with default value and select its row
  
          Triggered when the add button in the global variables tab is clicked.
          """
          gv_name = "new_global_%s" % self.global_variable_counter
          self.global_variable_counter += 1
          try:
              self.model.global_variable_manager.set_variable(gv_name, None)
          except (RuntimeError, AttributeError, TypeError) as e:
              logger.warning("Addition of new global variable '{0}' failed: {1}".format(gv_name, e))
          self.select_entry(gv_name)
          return True