title: R xlsx addDataFrame produces Error in sheet$getWorkbook : $ operator is invalid for atomic vectors
body: <p>I just installed the xlsx package and the requisite 64 bit Java for javaR behind it. I am trying to start with an incredibly simple operation: put an existing data frame into a spreadsheet. Every time I try to actually add the data frame to the worksheet, I get the error above. </p>
  
  <pre><code>TimingReport &lt;- createWorkbook() ##brand new workbook--seems fine
  createSheet(TimingReport,"Year2015") ##brand new sheet in workbook--seems fine
  addDataFrame(DataFromEarlier, "Year2015") ##add data frame--throws error
  </code></pre>
  
  <p>I checked the type on DataFromEarlier, and it is an actual data frame, not a matrix. But just in case something was wrong with it, I also tried making a clean demo data table.</p>
  
  <pre><code>data &lt;- data.frame(mon=c(1,2,3), day=c("m","w","f")) 
  addDataFrame(data, "Year2015")
  </code></pre>
  
  <p>Throws the same dang error! Any ideas as to what the underlying problem is and how to fix it?</p>