The compilation of the following MCE:
\documentclass{tikzposter}
% \font\nullfont=cmr10
\begin{document}
Foo
\end{document}
produces the following warnings:
Missing character: There is no F in font nullfont!
Missing character: There is no o in font nullfont!
Missing character: There is no o in font nullfont!
I tried to resort the \font\nullfont=cmr10 trick from this answer but I couldn't locate the culprit lines in the (up to date for a current TeX Live 2022 distribution: 2014/01/15 v2.0) tikzposter.cls file.
\node{Foo};. – Ulrike Fischer Feb 05 '23 at 13:40