In BaseWidget.destroy(), it removes self from its
master's dict of children before it calls the Tcl
destroy on the widget.  If the widget has a destroy
callback (like Togl, the Tk OpenGL widget), then
nametowidget throws an exception when given the
widget's name even though the Python widget still
exists.  Just reordering the code, so that the Tcl
destroy happens before the updating of the master's
dict of children, fixes the problem.

The bug is present in earlier versions of Tkinter too.