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This is mostly correct, and is fixed in the 8.4 Tcl sources 
already (I guess we can backport this).  This was mentioned 
in SF Tcl bug (account for chopped URL):

https://sourceforge.net/tracker/?
func=detail&aid=217982&group_id=10894&atid=110894

and the code comment is:

    /*
     * Only finalize the notifier if a notifier was 
installed in the
     * current thread; there is a route in which this is not
     * guaranteed to be true (when tclWin32Dll.c:DllMain() 
is called
     * with the flag DLL_PROCESS_DETACH by the OS, which 
could be
     * doing so from a thread that's never previously been 
involved
     * with Tcl, e.g. the task manager) so this check is 
important.
     *
     * Fixes Bug #217982 reported by Hugh Vu and Gene 
Leache.
     */
    if (tsdPtr == NULL) {
	return;
    }