Cancel the scheduled `function` calls.

def cancel(self, function):
        """Cancel the scheduled `function` calls."""
        if function in self._callback.keys():
            callback_id = self._callback[function][0]
            self.tk.after_cancel(callback_id)
            self._callback.pop(function)
        else:
            utils.error_format("Could not cancel function - it doesnt exist, it may have already run")