Add None handling for get_saved() in IDLE

[rhettinger]

BPO	35263
Nosy	@rhettinger, @terryjreedy
PRs	bpo-35379: Check IDLE objects before calling method  #10564
Superseder	bpo-35379: IDLE's close fails io is set to None on Mac

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/terryjreedy'
closed_at = <Date 2019-09-14.05:52:18.480>
created_at = <Date 2018-11-16.06:26:06.857>
labels = ['expert-IDLE', '3.7', '3.8']
title = 'Add None handling for get_saved() in IDLE'
updated_at = <Date 2019-09-14.05:52:18.479>
user = 'https://github.com/rhettinger'

bugs.python.org fields:

activity = <Date 2019-09-14.05:52:18.479>
actor = 'terry.reedy'
assignee = 'terry.reedy'
closed = True
closed_date = <Date 2019-09-14.05:52:18.480>
closer = 'terry.reedy'
components = ['IDLE']
creation = <Date 2018-11-16.06:26:06.857>
creator = 'rhettinger'
dependencies = []
files = []
hgrepos = []
issue_num = 35263
keywords = ['patch']
message_count = 2.0
messages = ['329982', '349439']
nosy_count = 2.0
nosy_names = ['rhettinger', 'terry.reedy']
pr_nums = ['10564']
priority = 'normal'
resolution = 'duplicate'
stage = 'resolved'
status = 'closed'
superseder = '35379'
type = None
url = 'https://bugs.python.org/issue35263'
versions = ['Python 3.7', 'Python 3.8']

[rhettinger]

Exception in Tkinter callback
Traceback (most recent call last):
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/tkinter/__init__.py", line 1705, in __call__
    return self.func(*args)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/idlelib/multicall.py", line 176, in handler
    r = l[i](event)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/idlelib/filelist.py", line 54, in close_all_callback
    reply = edit.close()
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/idlelib/pyshell.py", line 1014, in close
    return EditorWindow.close(self)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/idlelib/editor.py", line 1015, in close
    reply = self.maybesave()
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/idlelib/outwin.py", line 93, in maybesave
    return 'yes' if self.get_saved() else 'no'
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/idlelib/editor.py", line 948, in get_saved
    return self.undo.get_saved()
AttributeError: 'NoneType' object has no attribute 'get_saved'

[terryjreedy]

I suspect that this is related to bpo-35379, where I added some preliminary analysis.