Fix Issue #40 example worksheetoperations crash.

The error is caused by SHA: a1e53218bb

The "Sheet1" will not be cteated Unless the user want to access it,
or the user want to save an empty workbook.

examples/xlsx/worksheetoperations/main.cpp

@@ -5,12 +5,11 @@ int main()
 {
     QXlsx::Document xlsx;
 
-    xlsx.renameSheet("Sheet1", "TheFirstSheet");
-
     for (int i=1; i<20; ++i) {
         for (int j=1; j<15; ++j)
             xlsx.write(i, j, QString("R %1 C %2").arg(i).arg(j));
     }
+    xlsx.renameSheet("Sheet1", "TheFirstSheet");
 
     xlsx.addSheet("TheSecondSheet");
     xlsx.write(2, 2, "Hello Qt Xlsx");