Today the medallion atop the Tyn Church is an image of the Virgin Mary, a strictly Catholic image for this Catholic church.

When Jan Hus was alive and his movement flourishing, it was a different image: it was a chalice, referring to the Hussite practice of "communion in two species" (bread and wine).  For much of modern history, Catholic churches distributed communion with bread alone.  It is only in the very recent past that communion in two species has been (irregularly) practiced.