Missing character: There is no 5 ("35) in font nullfont LaTeX [Ln 1, Col 1]

Question

i write a testing slide by latex with beamer, but get some warnings:

Missing character: There is no 5 ("35) in font nullfont! LaTeX [Ln 1, Col 1]
Missing character: There is no 1 ("31) in font nullfont! LaTeX [Ln 1, Col 1]
Missing character: There is no 3 ("33) in font nullfont! LaTeX [Ln 1, Col 1]
Missing character: There is no 1 ("31) in font nullfont! LaTeX [Ln 1, Col 1]
Missing character: There is no 9 ("39) in font nullfont! LaTeX [Ln 1, Col 1]
Missing character: There is no 2 ("32) in font nullfont! LaTeX [Ln 1, Col 1]
Missing character: There is no 5 ("35) in font nullfont! LaTeX [Ln 1, Col 1]

How can I fix it?

My os is MacOS, and use xelatex engine.

I want to build a slide, and i use beamer, my engine is xelatex. The testing slide just contains some frames, and there are some testing contents in these frames.

The testing content is as follow:

\documentclass[12pt]{beamer}
\setbeamertemplate{bibliography item}[text]
\mode<presentation> 
{
\usetheme{Madrid}
}


\title[About Beamer] 
{About the Beamer class in presentation making}

\subtitle{A short story}

\author[Arthur, Doe]
{A.~B.~Arthur\inst{1} \and J.~Doe\inst{2}}

\institute[VFU] % (optional)
{
  \inst{1}%
  Faculty of Physics\\
  Very Famous University
  \and
  \inst{2}%
  Faculty of Chemistry\\
  Very Famous University
}

\date[VLC 2020] % (optional)
{Very Large Conference, April 2013}



\AtBeginSection[]
{
  \begin{frame}
    \frametitle{Table of Contents}
    \tableofcontents[currentsection]
  \end{frame}
}



\begin{document}

\frame{\titlepage}

\begin{frame}
\frametitle{Table of Contents}
\tableofcontents
\end{frame}


\section{Introduction}

\begin{frame}
\frametitle{Sample frame title}

\colorbox{orange}{highlight}

In this slide, some \textbf{important} text will be
\alert{highlighted} because it's important.
Please, don't \textcolor{green}{abuse} it. 

\begin{block}{Remark}
Sample text
\end{block}

\begin{alertblock}{Important theorem}
Sample text in red box
\end{alertblock}

\begin{examples}
Sample text in green box. The title of the block is ``Examples".
\end{examples}

\end{frame}


% Insert a thank you frame
\begin{frame}
\Huge{\centerline{Thank you!}}
\end{frame}

\end{document}