I should have thought of .idlerc as being a possible problem.  The usual fix is to delete its contents.

For this tracker, exiting with a traceback is a behavior issue; a crash is something worse, a core dump or whatever the MAC equivalent is.

.idlerc should not have config-main.cfg unless you change one of the values in idlelib/config-main.def.  If you change anything, the redundant entries will be removed.  Similarly for the other .idlerc/*.cfg versus idlelib/*.def files.

So missing that file should not have been a problem.  But it might have been if idlelib/config-main.def is corrupt.  I suggest you check it.  However, it it were, I would expect IDLE startup to fail, but it did not.

Since .idlerc is common to all installed version of Python, and hence to all IDLEs, it is not touched by installation or removal of any particular version.

The file you copied is perhaps a decade old.  However, the only changes should be

[EditorWindow]
font= TkFixedFont
[General]
print-command-posix=lpr %%s
print-command-win=start /min notepad /p %%s
[Theme]
name2=
# name2 set in user config-main.cfg for themes added after 2015 Oct 1

The third is needed if you select the new IDLE Dark color theme.

In ConfigDialog, self.fontSize is a StringVar, so it would seem that the error was it being '' rather than something like '10'.  Since you have 'fixed' the problem, I don't anticipate being to verify what its value was or why.  Hence I will close this.  However, feel free to post additional relevant information.  (Interpret 'Not a bug' here as 'cause unknown' ;-).