Toggles the timer for updating frozen cells

def OnTimerToggle(self, event):
        """Toggles the timer for updating frozen cells"""

        if self.grid.timer_running:
            # Stop timer
            self.grid.timer_running = False
            self.grid.timer.Stop()
            del self.grid.timer

        else:
            # Start timer
            self.grid.timer_running = True
            self.grid.timer = wx.Timer(self.grid)
            self.grid.timer.Start(config["timer_interval"])