MacOS下XeLaTeX编译Beamer幻灯片出现nullfont字符缺失警告的解决方法

在MacOS系统上使用XeLaTeX引擎编译Beamer测试幻灯片时，出现如下警告：

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]

测试代码如下：

\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}

解决方法 #

这个警告是XeLaTeX和Beamer部分模板交互时的小问题——系统尝试用nullfont输出数字字符，但nullfont本身不包含任何字符，所以触发警告。可以用以下几种方式解决：

• 注释掉无用的模板命令
  你代码里的\setbeamertemplate{bibliography item}[text]在没有添加参考文献的情况下是多余的，直接注释掉这行：

  % \setbeamertemplate{bibliography item}[text]
  

  重新编译后警告就会消失。

• 加载fontspec字体包
  在导言区添加fontspec包并指定系统可用字体，让XeLaTeX用正常字体处理这些字符：

  \usepackage{fontspec}
  \setmainfont{Times New Roman} % 换成你系统里有的字体，比如PingFang SC
  

• 更新TeX发行版
  旧版本的Beamer或XeLaTeX可能存在这类兼容性bug，把你的MacTeX（或其他TeX发行版）更新到最新版本，也能自动修复该问题。

内容的提问来源于stack exchange，提问作者XMan