fix issue borgbase#940 - KeyError in get_dict_from_list

- cause of error : defaultdict added more defaultdict while attempting to traverse the tree, but once a 'potential leaf' node was added, it was added as a dict, not a defaultdict.
- two possible solutions:
    - 1 - change everywhere that adds a 'potential' leaf node to add a defaultdict (ie nested_dict()) - this occurs in several places, but not many.
    - 2 - change get_dict_from_list to add a default dict (not defaultdict) when traversing the tree, for the case where a multi-level node is added on top of an existing node. This requires only changing a single location, and means that the dictionaries returned by accessing the tree will behave like normal dict (ie, won't by default add missing keys).

src/vorta/utils.py

@@ -1,13 +1,11 @@
 import argparse
 import errno
 import getpass
-import operator
 import os
 import platform
 import re
 import sys
 import unicodedata
-from collections import defaultdict
 from datetime import datetime as dt
 from functools import reduce
 
@@ -105,11 +103,11 @@ def nested_dict():
     - https://stackoverflow.com/a/16724937/3983708
     - https://stackoverflow.com/a/14692747/3983708
     """
-    return defaultdict(nested_dict)
+    return dict()
 
 
 def get_dict_from_list(dataDict, mapList):
-    return reduce(operator.getitem, mapList, dataDict)
+    return reduce(lambda d, k: d.setdefault(k, {}), mapList, dataDict)
 
 
 def choose_file_dialog(parent, title, want_folder=True):