A typo was introduced in commit https://git.xfce.org/xfce/libxfce4ui/commit/?id=6d4ec5e1ae3d515af32696323c6347e8943ce81a, line 55 NULL);f an errant "f".
Andre Miranda referenced this bugreport in commit c32fd5115cef980f057dcb5dbfcd98e894fd9bc5 Fix typo in test-ui.c (Bug #16253) https://git.xfce.org/xfce/libxfce4ui/commit?id=c32fd5115cef980f057dcb5dbfcd98e894fd9bc5
Nice catch, thanks for reporting!