Add protection make imported PDF page content become blank

[sonnht1409]

Hi

I do follow import the pdf page in the example and it works like a charm

package main

import (
    "bytes"
    "github.com/phpdave11/gofpdf"
    "github.com/phpdave11/gofpdf/contrib/gofpdi"
    "io"
    "io/ioutil"
    "net/http"
)

func main() {
    var err error

    pdf := gofpdf.New("P", "mm", "A4", "")

    // Download a PDF into memory                                                                                                                     
    res, err := http.Get("https://tcpdf.org/files/examples/example_038.pdf")
    if err != nil {
        panic(err)
    }
    pdfBytes, err := ioutil.ReadAll(res.Body)
    res.Body.Close()
    if err != nil {
        panic(err)
    }

    // convert []byte to io.ReadSeeker                                                                                                                
    rs := io.ReadSeeker(bytes.NewReader(pdfBytes))

    // Import in-memory PDF stream with gofpdi free pdf document importer                                                                             
    tpl1 := gofpdi.ImportPageFromStream(pdf, &rs, 1, "/TrimBox")

    pdf.AddPage()

    pdf.SetFillColor(200, 700, 220)
    pdf.Rect(20, 50, 150, 215, "F")

    // Draw imported template onto page                                                                                                               
    gofpdi.UseImportedTemplate(pdf, tpl1, 20, 50, 150, 0)

    pdf.SetFont("Helvetica", "", 20)
    pdf.Cell(0, 0, "Import PDF stream into gofpdf document with gofpdi")

    err = pdf.OutputFileAndClose("example.pdf")
    if err != nil {
        panic(err)
    }
}

But when I use SetProtection function, then all content of the imported page was gone
pdf.SetProtection(gofpdf.CnProtectAnnotForms, "1", "a")

As my demand is import a pdf page and set password for the pdf file, please help me walkthrough this