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Steps for reproduction:
1. Open IDLE (Python 3.1.2)
2. Open a .py file
3. With the code window (not the shell window) in focus, Ctrl + O to bring up the open file dialog. Do not select a file or press open. 
4. Close the code window. 
5. Select a file and try to open it.
6. The IDLE process will terminate. 
This test was performed on Windows 7 Professional 32-bit as well as Windows XP Professional 32-bit. 
Python 3.1.2 (r312:79149, Mar 21 2010, 00:41:52) [MSC v.1500 32 bit (Intel)] on win32

Reproduced in a console window:
C:\>c:\python31\python.exe -m idlelib.idle
Exception in Tkinter callback
Traceback (most recent call last):
 File "c:\python31\lib\tkinter\__init__.py", line 1399, in __call__
 return self.func(*args)
 File "c:\python31\lib\idlelib\IOBinding.py", line 177, in open
 self.editwin.flist.open(filename)
AttributeError: 'NoneType' object has no attribute 'flist'

That suggests that the last statement needs to be guarded somehow.
I re-versioned to 3.2 because 3.1.final will be out very soon.
I expect 3.2 and 2.7 should have same problem.

Ok. I'll see if I can get some protection around that then. 
I did test the issue with 2.7, and I didn't find it. The window didn't open, but it didn't generate an exception that would kill the IDLE process.

Ok, attached is a patch that should make IDLE silently ignore this happening; upon looking into this, it was a rather trivial fix. The open function was waiting on the return input from the askopenfile call, during which the calling window was closed, setting the editwin parameter to None according to close. The patch just adds another try/except to catch the AttributeError raised when the non-extant editwin's flist is referenced. 
I did come up with a method to actually make it continue with the opening process (just save a copy of the editwin's flist before the askopenfile call, during which the editwin gets closed), but that seemed a bit kludgey and possibly dangerous;, however it *seems* to work without issue. I can upload that patch as well if anyone would care to review it in addition to the attached patch.

The indentation of the patch 'looks' wrong. That appears to be because you used tabs instead of spaces (as in the lines removed and I presume elsewhere in the file -- and because FireFox interprets tabs as 8 spaces. Please redo with spaces. I will try to look at it during 3.2 beta phase if no one beats me to it.

Unexpected exception is not a crash. Changing the type to "behavior."

William's explanation in msg123203 for the cause of the error and the solution for keeping a reference to "flist" is good. IDLE has only one instance of FileList while running anyways.
Attached is a patch that behaves like William's description.
The modification to PyShell.py is necessary for a corner-case. If the last IDLE window is closed with the Open File dialog still open, then FileList.py and WindowList.py both call .quit(). Without the modification, IDLE closes after selecting a file since root.mainloop() returns and root.destroy() gets executed. 
The alternative (and simpler) method is to have the Open File dialog be modal, but AFAIK it can't be set to modal.

New changeset a2877fbabf95 by Terry Jan Reedy in branch '3.2':
Issue #10365: File open dialog now works instead of crashing
http://hg.python.org/cpython/rev/a2877fbabf95
New changeset 21862628a013 by Terry Jan Reedy in branch 'default':
Merge with 3.2
http://hg.python.org/cpython/rev/21862628a013
New changeset 4334964993b9 by Terry Jan Reedy in branch '2.7':
Issue #10365: File open dialog now works instead of crashing
http://hg.python.org/cpython/rev/4334964993b9
New changeset 7dae83057e83 by Terry Jan Reedy in branch '3.2':
#10365 Trim trailing whitespace
http://hg.python.org/cpython/rev/7dae83057e83
New changeset 630baa300173 by Terry Jan Reedy in branch '2.7':
#10365 Trim trailing whitespace
http://hg.python.org/cpython/rev/630baa300173 

I verified problem in 3.3 and when opening dialog from shell.
Patch fixes problem, including open idle, open dialog, close shell, select file, and now file is opened. Thanks Roger. I went ahead and applied fix. (But note: your patch had a 'blank' line with several spaces. Not allowed. We both should remember to run whitespace-at-end-of-line stripper.)
I did not close yet because I think the comments need improvement (and perhaps made into doc string). Hence 'needs (second) patch'.
 # If the current window has no filename and hasn't been
 # modified, we replace its contents (no loss). Otherwise
 # we open a new window. But we won't replace the
 # shell window (which has an interp(reter) attribute), which
 # gets set to "not modified" at every new prompt.
I think that means to say "If the current window is a never-modified new, empty, untitled edit window, load the file there." If it had any contents to replace, I believe it would be either titled or modified. So that seem misleading.
The 'otherwise..open a new window' is not true since a file can be opened (for writing) only once, and if it is already open, focus is simply shifted to its current window.
So I think the behavior can be summarized as "If the file is already open in an edit window, shift focus to that window. Otherwise open a new window (or use the current window if it is new, unmodified, and untitled) and load the file."
 # Also, make sure the current window has not been closed,
 # since it can be closed during the Open File dialog.
This seems misplaced where it is since overall we are *not* making sure that the current window is still open, but are taking measures to make sure open works even if it has been closed. I think that the comment that might be added before this block is that flist is being saved in case the parent window gets closed while the dialog is open.
I am also curious if you understand
 # Code for use outside IDLE:
or might this be obsolete?

(I need to write an extension to auto-apply strip-trailing-whitespace on save.)
Terry, I agree that the comment needs improvement. The self.editwin reference to the window needs to be available so that .get_saved() doesn't fail. In retrospect, the original patch obscured this point.
Attached is issue10365_rev3.patch, meant to be used against the latest code. It refactors the shell detection as part of the conditional as well as clarifies the comments.
The "Code for use outside IDLE" is part of a test at the end of the file which uses a duck-typed "EditorWindow" object. This object has flist=None which allows this extra code to run.

The only substantive change I see is replacing the four lines setting 'interp' with a defaulted getattr call. You seem to have missed my point that 'Otherwise, open a new window' is false. It must be that "flist.open(filename)" first checks to see if filename is open and if so, uses the existing window rather than opening a new one. (I would check this if I knew what class flist is and where it is defined.) This must be what happens with "flist.open(filename, self.loadfile)" as it loads the file into the existing window, renames itself, and updates the recent files list. I will make the code replacement and do some comment changes.

You're right. I missed your point about flist.open shifting focus to the already opened file. FileList.py contains the open method.

New changeset 5b267381eea0 by Terry Jan Reedy in branch '2.7':
Issue 10365: Add and replace comments; condense defaulted attribute access.
http://hg.python.org/cpython/rev/5b267381eea0
New changeset 4f3d4ce8ac9f by Terry Jan Reedy in branch '3.2':
Issue 10365: Add and replace comments; condense defaulted attribute access.
http://hg.python.org/cpython/rev/4f3d4ce8ac9f
New changeset d9b7399d9e45 by Terry Jan Reedy in branch 'default':
Merge with 3.2 #10365
http://hg.python.org/cpython/rev/d9b7399d9e45 

Forgive me if I'm not following the correct process. But I believe I have seen this issue again in 3.3. Not sure I captured exactly what is needed from the command line. The file being openeed is from the Python Hands On Class Examples http://anh.cs.luc.edu/python/hands-on/
c:\python33\python.exe -m idlelib.idle
Exception in Tkinter callback
Traceback (most recent call last):
 File "c:\python33\lib\tkinter\__init__.py", line 1442, in __call__
 return self.func(*args)
 File "c:\python33\lib\idlelib\MultiCall.py", line 174, in handler
 doafterhandler.pop()()
 File "c:\python33\lib\idlelib\MultiCall.py", line 221, in <lambda>
 doit = lambda: self.bindedfuncs[triplet[2]][triplet[0]].remove(func)
ValueError: list.remove(x): x not in list
Traceback (most recent call last):
 File "c:\python33\lib\runpy.py", line 160, in _run_module_as_main
 "__main__", fname, loader, pkg_name)
 File "c:\python33\lib\runpy.py", line 73, in _run_code
 exec(code, run_globals)
 File "c:\python33\lib\idlelib\idle.py", line 11, in <module>
 idlelib.PyShell.main()
 File "c:\python33\lib\idlelib\PyShell.py", line 1477, in main
 root.mainloop()
 File "c:\python33\lib\tkinter\__init__.py", line 1038, in mainloop
 self.tk.mainloop(n)

It seems like a logic error to try to remove something that is not there. But it is not obvious from the traceback that your problem has anything to do with *opening* a file. Unbinding should only happen when *closing* a file. So I suspect this is a different issue.
Please
1. copy and paste the 'sign-in' line that looks like this:
"Python 3.3.0 (v3.3.0:bd8afb90ebf2, Sep 29 2012, 10:57:17) [MSC v.1600 64 bit (AMD64)] on win32"
2. add the OS version (like 'Win 7')
3. say *exactly* what you do and what happens when. In particular, do you get the traceback when you close a file? Or only when you actually press the [Open] button.
Try different python files to make sure it is not a problem with the specific file. Does your IDLE otherwise seem to run normally?

Patrick, see Issue8900. It described your problem.

Thanks for the pointer to the other issue. It looks spot on.