title: Officer - Error when adding a level 1 paragraph onto existing slide
body: <p>I'm trying to write a sequence of bullet points onto the same slide, with the following R-Code:</p>
  
  <pre><code>library(officer)
  library(magrittr)
  
  ppt &lt;- read_pptx()
  
  ppt &lt;- ppt %&gt;%
      add_slide(layout = "Title and Content", master = "Office Theme") %&gt;%
      ph_with_text(type = "title", str = "FFT power spectrum") %&gt;%
      ph_with_text(type = "body", str = "no visual discrimination") %&gt;%
      ph_add_par(level = 1L) %&gt;%
      ph_add_text(str = "whole trajectories")
  
  
  print(ppt, target = "test.pptx") %&gt;% invisible()
  </code></pre>
  
  <p>... which fails, saying ...</p>
  
  <pre><code>Error in doc_parse_raw(x, encoding = encoding, base_url = base_url, as_html = as_html,  : 
    Premature end of data in tag p line 1 [77]
  </code></pre>
  
  <p>When I change the level-parameter to</p>
  
  <pre><code>ph_add_par(level = 2L)
  </code></pre>
  
  <p>it works just fine (as demonstrated in the vignette: <a href="https://cran.r-project.org/web/packages/officer/vignettes/powerpoint.html" rel="nofollow noreferrer">https://cran.r-project.org/web/packages/officer/vignettes/powerpoint.html</a>)</p>
  
  <p>What am I missing here?</p>