We use the so called valuespec ListOf() in several places where a dynamic amount
of elements can be added to a list of things, for example the number of exceptions
of a timeperiod in the timeperiod edit dialog.
Now, when you created a new timeperiod, added one exception and imediately removed
it, you had no chance to save this timeperiod without adding another exception to
the list. This has been fixed now.
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