The package hf-tikz is great for putting a frame around an equation. Although I am not absolutely sure there seems to be a "new" error massage when using this package: "Missing character: There is no ; in font nullfont!"
I stripped my complex beamer-file down to actually a minimal LaTeX-file which also gives the error message:
c^2 = a^2+b^2
The resulting PDF-file is ok with no errors, but the error message makes me a bit nervous because the question is always what happens after the next update? My system is a MINT Linux 21.1 with an underlying Ubuntu 22.04 and a full installation of Texlive 2022.
If I uncomment the line "% \font\nullfont=cmr10" a semicolon appears at the end of the coloured box. From other postings about the "Missing character..." error message I learned that setting the nullfont to a visible font allows to find the error in the underlying LaTeX code. It seems that there is an error in the package hf-tikz or that the syntax of tikz has changed a little bit and the older package hf-tikz has not been adapted to these changes.
Any idea how to overcome the problem?