What is the correct way to append Content control to a newly created docx file using c# and openxml

This is my code.

using (WordprocessingDocument package = WordprocessingDocument.Create(docName, WordprocessingDocumentType.Document)) {

            MainDocumentPart mainPart = package.AddMainDocumentPart();
            mainPart.Document = new Document();
            Body body = new Body();

            SdtBlock sdtBlock1 = new SdtBlock();

            SdtProperties sdtProperties1 = new SdtProperties();
            SdtAlias sdtAlias1 = new SdtAlias() { Val = "select" };
            Tag tag1 = new Tag() { Val = "select" };
            SdtId sdtId1 = new SdtId() { Val = 364947247 };

            SdtContentComboBox sdtContentComboBox1 = new SdtContentComboBox();
            ListItem listItem1 = new ListItem() { Value = "Choose an item." };
            ListItem listItem2 = new ListItem() { DisplayText = "Rama", Value = "Rama" };
            sdtContentComboBox1.Append(listItem1);
            sdtContentComboBox1.Append(listItem2);
            sdtProperties1.Append(sdtAlias1);
            sdtProperties1.Append(sdtContentComboBox1);

            SdtContentBlock sdtContentBlock1 = new SdtContentBlock();

            Run run1 = new Run();
            Text text1 = new Text();
            text1.Text = "Rama";

            run1.Append(text1);
            sdtBlock1.Append(sdtProperties1);
            sdtBlock1.Append(sdtContentBlock1);
            mainPart.Document.Body.Append(sdtBlock1);
        } 

I am trying to create a document and add a Dropdownlist content control to it but when I run this code VS Debugger is stops the program saying "Null Reference Exception was unhandled". I want to know How to append or insert a new content control on a newly created document.

Thanks in advance.