Description of problem:
I opened a two large directories in meld.

Version-Release number of selected component:
meld-3.19.0-3.fc29

Additional info:
reporter:       libreport-2.9.5
cmdline:        /usr/bin/python3 /usr/bin/meld ../new_frontend_project .
crash_function: inorder_search_down
exception_type: StopIteration
executable:     /usr/bin/meld
interpreter:    python3-3.7.0-9.fc29.x86_64
kernel:         4.18.5-300.fc29.x86_64
runlevel:       N 5
type:           Python3
uid:            1000

Truncated backtrace:
treehelpers.py:127:get_previous_next_paths:RuntimeError: generator raised StopIteration

Traceback (most recent call last):
  File "/usr/lib/python3.7/site-packages/meld/treehelpers.py", line 87, in inorder_search_down
    raise StopIteration()
StopIteration

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/usr/lib/python3.7/site-packages/meld/dirdiff.py", line 1090, in on_treeview_cursor_changed
    prev, next = self.model._find_next_prev_diff(cursor_path)
  File "/usr/lib/python3.7/site-packages/meld/tree.py", line 173, in _find_next_prev_diff
    return self.get_previous_next_paths(start_path, match_func)
  File "/usr/lib/python3.7/site-packages/meld/treehelpers.py", line 127, in get_previous_next_paths
    for it in self.inorder_search_down(start_iter):
RuntimeError: generator raised StopIteration

Local variables in innermost frame:
self: <dirdiff.DirDiffTreeStore object at 0x7f1210d19240 (meld+dirdiff+DirDiffTreeStore at 0x5648eba231a0)>
path: <Gtk.TreePath object at 0x7f1202a3c228 (GtkTreePath at 0x5648ec788e80)>
match_func: <function DiffTreeStore._find_next_prev_diff.<locals>.match_func at 0x7f1210d53d08>
prev_path: <Gtk.TreePath object at 0x7f1202a3be28 (GtkTreePath at 0x5648ec789000)>
next_path: None
start_iter: <Gtk.TreeIter object at 0x7f1202a3b8e8 (GtkTreeIter at 0x5648eb244560)>
it: <Gtk.TreeIter object at 0x7f1202a3be88 (GtkTreeIter at 0x5648eb243f60)>