Fix for uimenu crash (#6214)

Turns out that ncurses `werase` tolerates a `NULL` pointer (through the man page says it returns an error, but who checks returns codes anyway?).

The tiles version of `werase` does not.

Btw. the pointer is empty because the uimenu created has no entries to display and therefor skips the display (and creation) of a window. And the error only happens when one does not enter any text. The other crash happened only when one did enter text. That is confusing.

Fixes #6214