This closes #714 and closes #715, fix wrong worksheet index returned by NewSheet in some case, fix panic on formatted value with no built-in number format ID

cell.go

@@ -770,6 +770,9 @@ func (f *File) formattedValue(s int, v string) string {
 	if ok != nil {
 		return ok(v, builtInNumFmt[numFmtId])
 	}
+	if styleSheet == nil || styleSheet.NumFmts == nil {
+		return v
+	}
 	for _, xlsxFmt := range styleSheet.NumFmts.NumFmt {
 		if xlsxFmt.NumFmtID == numFmtId {
 			format := strings.ToLower(xlsxFmt.FormatCode)

cell_test.go

@@ -299,4 +299,14 @@ func TestFormattedValue(t *testing.T) {
 	assert.NoError(t, err)
 	v = f.formattedValue(1, "43528")
 	assert.Equal(t, "03/04/2019", v)
+
+	// formatted value with no built-in number format ID
+	assert.NoError(t, err)
+	f.Styles.NumFmts = nil
+	numFmtID := 5
+	f.Styles.CellXfs.Xf = append(f.Styles.CellXfs.Xf, xlsxXf{
+		NumFmtID: &numFmtID,
+	})
+	v = f.formattedValue(1, "43528")
+	assert.Equal(t, "43528", v)
 }

sheet.go

@@ -37,8 +37,9 @@ import (
 // appending the new sheet.
 func (f *File) NewSheet(name string) int {
 	// Check if the worksheet already exists
-	if f.GetSheetIndex(name) != -1 {
-		return f.SheetCount
+	index := f.GetSheetIndex(name)
+	if index != -1 {
+		return index
 	}
 	f.DeleteSheet(name)
 	f.SheetCount++

sheet_test.go

@@ -71,6 +71,8 @@ func TestNewSheet(t *testing.T) {
 	// delete original sheet
 	f.DeleteSheet(f.GetSheetName(f.GetSheetIndex("Sheet1")))
 	assert.NoError(t, f.SaveAs(filepath.Join("test", "TestNewSheet.xlsx")))
+	// create new worksheet with already exists name
+	assert.Equal(t, f.GetSheetIndex("Sheet2"), f.NewSheet("Sheet2"))
 }
 
 func TestSetPane(t *testing.T) {