docstr: Stop the timer.
  Unschedule the previous callback if it exists.
code: def _vispy_stop(self):
      """Stop the timer.
      Unschedule the previous callback if it exists.
      """
      if self._id is not None:
          self._tk.after_cancel(self._id)
          self._id = None
  
imports: from __future__ import division
  from time import sleep
  import warnings
  from ..base import BaseApplicationBackend, BaseCanvasBackend, BaseTimerBackend
  from ...util import keys
  from ...util.ptime import time
  from ...gloo import gl
label_desc: Use image node_id 3 for calling the TimerBackend obj's underlying member method code with example usage: obj._vispy_stop() without return types