title: Conditional formatting giving AttributeError: 'NoneType' object has no attribute 'group'
body: <p>I am trying to run a conditional format for one of my column in excel using xlswriter. I want to have the blank rows in the column in a certain format. Below I have attached the code snippet. But it is giving me an AttributeError:'NoneType' object has no attribute 'group'.</p>
  <pre class="lang-py prettyprint-override"><code>format1 = workbook.add_format({'bg_color': '#FFC7CE',
                                 'font_color': '#9C0006'})
  
  worksheet.conditional_format('G:H', {'type': 'no_blanks','format':format1})
  </code></pre>
  <p>Can anyone help me on this?</p>