Tcl/Tk uses modified utf-8 internally.  This includes using 0xC080, a multibyte Unicode null character, for embedded nulls that work with C's null terminated strings.  Java does the same.

Note that typing Ctrl-space and Ctrl-2 are conventional ways to enter a null from the keyboard.  That's the reason a null char is associated with those key combinations.

When Tcl exports Unicode, it is supposed to be strict utf-8.  Until Tcl8.5, the %A (Unicode character corresponding to an event) was incorrectly leaking the modified Unicode null.

_tkinter.c.2.patch is narrowly focused: if PythonCmd raises a UnicodeDecodeError and if the string passed in an arg is 0xC080, it is replaced with the Unicode null 0x00.