Beamer SubSubSection Ifx 无法正常工作

Question

如何使 ifx subsubsection 在下面的代码中工作。我希望代码仅在省略 subsubsection 时才进行子部分处理。

\documentclass{beamer}
\usetheme[sectionpage=none, progressbar=frametitle, numbering=fraction]{metropolis} 

\makeatletter
\AtBeginSubsubsection[]{%
\begin{frame}
  \centering
  \begin{minipage}{22em}
\raggedright
\usebeamercolor[fg]{section title}
\usebeamerfont{section title}
\insertsectionhead\\[-1ex]
\usebeamertemplate*{progress bar in section page}
\par
\ifx\insertsubsubsectionhead\@empty
     \usebeamercolor[fg]{subsection title}%
    \usebeamerfont{subsection title}%
    \insertsubsectionhead{} <-- This is not working
\else% 
  \usebeamercolor[fg]{subsection title}%
  \usebeamerfont{subsection title}%
  \insertsubsectionhead{} -- \insertsubsubsectionhead
\fi
\end{minipage}
\par
 \vspace{\baselineskip}
\end{frame}
 }
 \makeatother

\begin{document}

\section{section title}
\subsection{subsection title}


\begin{frame}
\frametitle{Title}

\end{frame}

\section{section title}
\subsection{subsection title}
\subsubsection{subsubsection title}

\end{document}