docstr: Return the Tk instance.
  
  Returns
  -------
  tk.Tk
      The tk.Tk instance.
code: def get_tk_instance(cls):
      """Return the Tk instance.
  
      Returns
      -------
      tk.Tk
          The tk.Tk instance.
      """
      if cls._tk_inst is None:
          if tk._default_root:
              # There already is a tk.Tk() instance available
              cls._tk_inst = tk._default_root
              cls._tk_inst_owned = False
          else:
              # Create our own top level Tk instance
              cls._tk_inst = tk.Tk()
              cls._tk_inst.withdraw()
              cls._tk_inst_owned = True
      return cls._tk_inst
  
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 1 for calling the _TkInstanceManager obj's underlying member method code with example usage: obj.get_tk_instance(cls) and returns: cls