This probably isn't a traceback that's likely to happen, but I wanted to document it since I was able to recreate it.

To recreate:
In a new shell, do Select All, then Find Selection.

Exception in Tkinter callback
Traceback (most recent call last):
  File "N:\projects\cpython\lib\tkinter\__init__.py", line 1883, in __call__
    return self.func(*args)
  File "N:\projects\cpython\lib\idlelib\editor.py", line 644, in find_selection_event
    search.find_selection(self.text)
  File "N:\projects\cpython\lib\idlelib\search.py", line 25, in find_selection
    return _setup(text).find_selection(text)
  File "N:\projects\cpython\lib\idlelib\search.py", line 72, in find_selection
    return self.find_again(text)
  File "N:\projects\cpython\lib\idlelib\search.py", line 65, in find_again
    self.bell()
AttributeError: 'SearchDialog' object has no attribute 'bell'

Thanks for reporting this. It is easy to trigger. And after a bit of analysis, easy to fix.

self.bell is set as top.bell in searchbase.SearchDialogBase.create_widgets(). The latter is called by SDB.open(), which is called for Find, Find-again if there is no previously defined pattern, and Find-in-files. It is not called by Find-selection as it uses the selection as the pattern.

The bell only sounds if the selection is not found anywhere else in the editor.
if selfirst == first and sellast == last:
self.bell()
If the selection is found elsewhere, the copy is then tagged with the selection tag, so when the original is found, the condition is not true. I consider it a bug that the selection tag is used instead of the 'found' tag, but the above depends on it being moved.

The purpose of creating and calling self.bell instead of top.bell is so bell can be mocked when testing, both to detect the call and suppress noise when testing. Binding self.bell to root.bell in __init__ instead of to top.bell in create_widgets should work fine.

I expect that people have run into this. But since callback exceptions do not crash IDLE, the only symptom when not running from a terminal is the lack of the 'bell' sound.

New changeset c465682 by Terry Jan Reedy in branch 'master':
bpo-35591: IDLE Find Selection now works when selection not found (GH-11339)
c465682

New changeset 098bb24 by Miss Islington (bot) in branch '3.7':
bpo-35591: IDLE Find Selection now works when selection not found (GH-11339)
098bb24

Thanks for looking at this one. :-)