cb.current() not working when only one Checkbox selected

[DavidVoellm]

When only one CheckBox is selected cb.current() throws an Error, because self.get() returns the selected value as a String and not as array.
To solve this self.get() needs to be checked if is String and converted to array.

Here: (in method current)

        retarr = self.get()
        if type(retarr) == str: retarr = [retarr] # EDIT BY DABVIT
        if len(retarr) == 0:
            return -1
        elif len(retarr) == 1:
            if retarr[0] == '':
                return -1
            else:
                return self.cget('values').index(retarr[0])
        else:
            return [self.cget('values').index(i) for i in retarr] # EDIT BY DABVIT