Vminibuf_selected_window

[Richard Stallman]

If it isn't a Lisp variable, it is better to remove the V from the
name so people won't think it is supposed to be a Lisp variable.

Meanwhile, I think you need to staticpro that variable
and initialize it at startup, in syms_of_window.

If you can't prove the window is always pointed to elsewhere,
then the staticpro is needed.  

If you can prove that the window cannot remain there
without being pointed to and thus preserved by other live data,
then the staticpro is not necessary, but you should state
the reasoning in a comment next to the variable.  However,
it might be safer to put in the staticpro anyway.