The signature for tkinter class Variable and its subclasses StringVar, IntVar, DoubleVar, BooleanVar is
def __init__(self, master=None, value=None, name=None):
However, the None default is invalid because of
self._tk = master.tk
The preceding lines
if not master:
master = _default_root
might suggest that None is acceptible. But _default_root is set to None at the top of the file, so that just replaces None with None.

If Variables must have a gui widget parent, then they cannot be used in gui-free tests, even though they have no graphic representation.

Moveover, not everything is a proper master. configSectionNameDialog.py . GetCfgSectionNameDialog(Toplevel) . Create.Widgets(self) has these lines:
self.name = StringVar(self)
self.fontSize = StringVar(self)
However, these are not (always) garbage collectable.

After fixing the dialog test at the bottom of the file, and running it with 3.3 python_d -m idlelib.configSectionNameDialog (bpo-18130), and adding the gc flag, I repeatedly got
[sys:1: ResourceWarning: gc: 2 uncollectable objects at shutdown [<tkinter.StringVar object at 0x02C597E0>, <tkinter.StringVar ...]]
(This only showed when running from the console, not when running within Idle started within the interactive debug interpreter.)

Running the test multiple times within a session, by repeatedly creating new dialogs with the [dialog] button in the test window, multiplied the uncollected objects.

Replacing 'self' with 'self.parent' solved the problem, at least in the test situation.

Just a gentle reminder.

My first message discussed two different issues. The first, generalized, is that unconditionally using tkinter._default_root, which can be either None or absent, as a backup for master=None in __init__ functions, seems a bit sloppy. Image.__init__ does this check
if not master:
master = _default_root
if not master:
raise RuntimeError('Too early to create image')
but that will fail if _default_root has been deleted. Also, the message is wrong in that it is not 'too early' if an explicit master is passed. I am thinking that all uses of _default_root should raise something like WhateverError("An explicit master is required when _default_root is None or deleted.") Serhiy, what do you think?

The second issue noted that while widgets often have a container widget as master, the non-graphics classes like Variable and Font should have a Tk and not a widget as master. I have corrected idlelib.configHandler and there is a tkinter patch on another issue (applied yet?) to automatically replace master=widget by master-widget.tk. So Variable calls are no longer part of this issue.

I am thinking that all uses of _default_root should raise something like
WhateverError("An explicit master is required when _default_root is None or
deleted.") Serhiy, what do you think?

Technically all works (raises some exception), but I agree that error message
can be more friendly. But _default_root is implementation detail. This name
doesn't mean anything for non-experienced user (and experienced user already
knows about these mystical RuntimeError and NameError). The message should say
something like "No default root window" if _default_root is None and "The
master argument is mandatory after calling NoDefautRoot()" if _default_root
isn't set.

And now we came to the way how to create default root window. bpo-4343 looks
related (I haven't looked at it close however).

Second issue was fixed in bpo-22068.

And the first issue was fixed in bpo-42630.