The issue #675 asks for dropping the string initialization of the Tree. So, these two still work:

Tree('dp', [Tree('d', ['the']), Tree('np', ['dog'])])
Tree.fromstring('(dp (d the) (np dog))')

But this not:
Tree('(dp (d the) (np dog))')

So now we can't initialize a Tree by supplying it a string (line 44 will break).