Include chapters in List of Figures fails for appendices

Question

I add contents lines containing the chapter numbers in the list of figures of a document, by using the third answer of the following question:

Include chapters in List of Figures with titletoc?

All is good except when I add an appendix, where I have the following error message:

! Missing number, treated as zero.! You can't use \numexpr in horizontal mode. \ifnumcomp ...\ifnum \numexpr #1\relax #2\numexpr...

but it compiles good ! so it gives the desired result despite the error message !

What's the cause of this error message, and how can I avoid it ?

I give the original file, where I only add the command for appendix:

\documentclass{book}
\usepackage{etoolbox}
\usepackage[toc,page]{appendix}  %% ADDED TO THE ORIGINAL FILE


\makeatletter
\def\thischaptertitle{}
\apptocmd{@chapter}{\gdef\thischaptertitle{#1}}{}{}
\newcommand{\DeclareDividedList}[1]%
  {\newcounter{#1@chapter}\setcounter{#1@chapter}{0}}
\pretocmd{\addcontentsline}%
  {\ifltxcounter{#1@chapter}%
   {%
     \ifnumgreater{\thechapter}{\value{#1@chapter}}{%
       \setcounter{#1@chapter}{\thechapter}%
       \addtocontents{#1}{\protect\contentsline{chapter}%
         {\protect\numberline {\thechapter} {\thischaptertitle}}{}{} }
     }{}%
   }{}%
  }{}{}
\makeatother
\DeclareDividedList{lof}
\DeclareDividedList{lot}
\usepackage[paperheight=12cm,vscale=0.9]{geometry}
\begin{document}
\tableofcontents
\listoffigures
\listoftables
\mainmatter
\chapter{Introduction with no Figures}
\chapter{Test Chapter with Figures but no Tables}
\begin{figure}
\caption{caption text}
\end{figure}
\begin{figure}
\caption{caption text}
\end{figure}
\chapter{Test Chapter with Tables but no Figures}
\begin{table}
\caption{caption text}
\end{table}
\begin{table}
\caption{caption text}
\end{table}
\chapter{Test Chapter with Figures and Tables}
\begin{figure}
\caption{caption text}
\end{figure}
\begin{table}
\caption{caption text}
\end{table}
\begin{figure}
\caption{caption text}
\end{figure}
\begin{table}
\caption{caption text}
\end{table}
\begin{figure}
\caption{caption text}
\end{figure}
%%% ADDED APPENDIX

    \begin{appendices}
    \renewcommand{\chaptername}{Appendix}
    \chapter{Appendix One}
    \begin{figure}
    \caption{Figure 1 of appendix 1}
    \end{figure}
    \end{appendices}
\end{document}

and the result obtained:

Answer 1

tl;dr: When you're using a counter in a comparison or to set a counter, you want to use \value{counter}, not \thecounter. And watch out for counters resetting in the appendix.

\thecounter prints a text representation of that counter, but this is not necessarily a number (for example, the chapter becomes A). This means that commands that expect a number see a letter instead ("Missing number..."), which led to your output having a spurious "A¿A" just before the caption.

But once you use \value{counter}, you run into an additional problem: because the chapter numbering starts over in the appendix, your \ifnumgreater no longer sees a larger number, and therefore doesn't write \thischaptertitle. The solution is to reset the divided counter. This leads to:

\documentclass{book}
\usepackage{etoolbox}
\usepackage[toc,page]{appendix}  %% ADDED TO THE ORIGINAL FILE


\makeatletter
\def\thischaptertitle{}
\apptocmd{@chapter}{\gdef\thischaptertitle{#1}}{}{}
\newcommand{\DeclareDividedList}[1]{%
  \newcounter{#1@chapter}%
  \setcounter{#1@chapter}{0}%
  \AtBeginEnvironment{appendices}{\setcounter{#1@chapter}{0}}% added
}
\pretocmd{\addcontentsline}%
  {\ifltxcounter{#1@chapter}%
   {%
     \ifnumgreater{\value{chapter}}{\value{#1@chapter}}{% \the -> \value{}
       \setcounter{#1@chapter}{\value{chapter}}% \the -> \value{}
       \addtocontents{#1}{\protect\contentsline{chapter}%
         {\protect\numberline {\thechapter} {\thischaptertitle}}{}{} }
     }{}%
   }{}%
  }{}{}
\makeatother
\DeclareDividedList{lof}
\DeclareDividedList{lot}
\usepackage[paperheight=12cm,vscale=0.9]{geometry}
\begin{document}
\tableofcontents
\listoffigures
\listoftables
\mainmatter
\chapter{Introduction with no Figures}
\chapter{Test Chapter with Figures but no Tables}
\begin{figure}
\caption{caption text}
\end{figure}
\begin{figure}
\caption{caption text}
\end{figure}
\chapter{Test Chapter with Tables but no Figures}
\begin{table}
\caption{caption text}
\end{table}
\begin{table}
\caption{caption text}
\end{table}
\chapter{Test Chapter with Figures and Tables}
\begin{figure}
\caption{caption text}
\end{figure}
\begin{table}
\caption{caption text}
\end{table}
\begin{figure}
\caption{caption text}
\end{figure}
\begin{table}
\caption{caption text}
\end{table}
\begin{figure}
\caption{caption text}
\end{figure}
%%% ADDED APPENDIX

    \begin{appendices}
    \renewcommand{\chaptername}{Appendix}
    \chapter{Appendix One}
    \begin{figure}
    \caption{Figure 1 of appendix 1}
    \end{figure}
    \end{appendices}
\end{document}