Fst gives a NoneType error when no indices are given (the default)

I have a simulation that does the right thing when I explicitly specify pairs of indices:

ts.Fst(sample_sets = samples_list,
      indexes = [(0,2), (0,1)])

outputs

array([[0.12508121, 0.12780193]])

but that gives the following error if I remove the indexes input (and so use the default value of None)

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-6-fbfb01290311> in <module>
----> 1 ts.Fst(sample_sets = samples_list)

/usr/local/lib/python3.7/site-packages/tskit/trees.py in Fst(self, sample_sets, indexes, windows, mode, span_normalise)
   3332         fst = np.repeat(1.0, np.product(divergences.shape))
   3333         fst.shape = divergences.shape
-> 3334         for i, (u, v) in enumerate(indexes):
   3335             denom = (diversities[:, :, u] + diversities[:, :, v]
   3336                      + 2 * divergences[:, :, i])

TypeError: 'NoneType' object is not iterable

[petrelharp]

Whoops; missed this. Let’s discuss at #315 .

[jeromekelleher]

I’ve had a change of heart here, and think maybe we should hold off on specifying a default for this—see the comment in #313