title: Miktex did not succeed after I updated R to version 3.5.0
body: <p>After I updated R to version 3.5.0, I couldn't start my MikTex from Rstudio,</p>
  
  <pre><code>---
  output:
    pdf_document:
      latex_engine: xelatex 
      includes:
        in_header: header.tex
      keep_tex: yes
  ---
  </code></pre>
  
  <p>Above is the code I typed ahead of my Rmd, and after knitring,</p>
  
  <pre><code>Sorry, but D:\miktex\miktex\bin\x64\xelatex.exe did not succeed.
  
  The log file hopefully contains the information to get MiKTeX going again:
  
    C:/Users/DELL/AppData/Local/MiKTeX/2.9/miktex/log/xelatex.log
  </code></pre>
  
  <p>Information showed in xelatex.log:</p>
  
  <pre><code>2018-04-29 10:06:17,482+0800 FATAL xelatex - Info: 
  2018-04-29 10:06:17,482+0800 FATAL xelatex - Source: Libraries\MiKTeX\UI\Qt\mikuiqt.cpp
  2018-04-29 10:06:17,482+0800 FATAL xelatex - Line: 45
  2018-04-29 10:06:17,482+0800 INFO  xelatex - finishing with exit code 1
  2018-04-29 10:06:17,626+0800 INFO  xelatex - starting with command line: D:\miktex\miktex\bin\x64\xelatex.exe -halt-on-error -interaction=batchmode 1.tex
  2018-04-29 10:06:17,631+0800 INFO  xelatex - allowing known shell commands
  2018-04-29 10:06:17,632+0800 INFO  xelatex - enabling input (output) from (to) processes
  2018-04-29 10:06:17,812+0800 FATAL xelatex - GUI framework cannot be initialized.
  </code></pre>
  
  <p>And after I tried:</p>
  
  <blockquote>
    <p>Sys.which("xelatex")</p>
  </blockquote>
  
  <pre><code>                                    xelatex 
  "D:\\miktex\\miktex\\bin\\x64\\xelatex.exe"
  </code></pre>
  
  <blockquote>
    <p>Sys.setenv("PATH" = "D:\miktex\miktex\bin\x64\xelatex.exe" )</p>
  </blockquote>
  
  <p>then knitr again:</p>
  
  <pre><code>Warning messages:
  
  1: In system2(..., stdout = FALSE, stderr = FALSE) : '"xelatex"' not found
  2: In system2(...) : '"xelatex"' not found
  
  No TeX installation detected (TeX is required to create PDF output). You should install a recommended TeX distribution for your platform:
  
    Windows: MiKTeX (Complete) - http://miktex.org/2.9/setup
    (NOTE: Be sure to download the Complete rather than Basic installation)
  </code></pre>
  
  <p>Could anyone help? Thanks.</p>