title: NPOI Core - Formula Issue
body: <p>I am trying to create a simple formula of <code>SUM(E1:E6)</code> but keep getting a <code>Value cannot be null. Parameter name: stream</code>.  I have put an <code>=</code> in front and even made it lower case with no success.  A formula I was able to get to work was <code>1 + 3 + 8</code>.  I then tried <code>E1 + E2 + E3 + E4 + E5 + E6</code> and that gave the same error.</p>
  
  <p>Here is my code:</p>
  
  <pre><code>IWorkbook workbook = new XSSFWorkbook();
  ISheet sheet1 = workbook.CreateSheet("Project Time");
  
  ... Other numeric and textual cells fill in and work great
  
  row = sheet1.CreateRow(RowIndex++);
  
  var test = row.CreateCell(ColIndex++);
  test.SetCellType(CellType.Formula);
  test.SetCellFormula($"1 + 2 + 3 + 4 + 5 + 6"); // WORKS
  test.SetCellFormula($"SUM(1 + 2 + 3 + 4 + 5 + 6)"); // FAILS
  test.SetCellFormula($"SUM(E1:E6)"); // FAILS
  test.SetCellFormula($"E1 + E2 + E3"); // FAILS
  
  XSSFFormulaEvaluator.EvaluateAllFormulaCells(workbook);
  
  workbook.Write(TheStream);
  </code></pre>
  
  <p>The GitHub location <a href="https://github.com/dotnetcore/NPOI" rel="nofollow noreferrer">is here</a>.</p>