title: Rtexttools Trouble creating document term matrix with create_matrix
body: <p>I'm using RTextTools for the first time. Here's my code for create_matrix</p>
  
  <pre><code>library(RTextTools)
  texts &lt;- c("This is the first document.", 
            "Is this a text?", 
          "This is the second file.", 
          "This is the third text.", 
          "File is not this.") 
  doc_matrix &lt;- create_matrix(texts, language="english", removeNumbers=FALSE, stemWords=TRUE, removeSparseTerms=.2)
  </code></pre>
  
  <p>I'm getting the following error(s):</p>
  
  <pre><code>Error in `[.simple_triplet_matrix`(matrix, , sort(colnames(matrix))) : 
  Invalid subscript type: NULL.
  In addition: Warning messages:
  1: In is.na(x) : is.na() applied to non-(list or vector) of type 'NULL'
  2: In is.na(j) : is.na() applied to non-(list or vector) of type 'NULL'
  </code></pre>
  
  <p>I haven't seen anyone else post this error yet, and figure there's something very basic which I am missing.</p>
  
  <p>Peter</p>