how to handle "NullReferenceException was unhandled by user code" for FileUpload in C#

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C#

.NET

I am doing a sample project for an internship in a company.
According to the project the New user have to register their account in the registration screen..
there we have an option to upload our profile picture...
I have mentioned the photo upload codings below....

protected void NextLinkbutton3_OnClick(object sender, EventArgs e)
{
Session["ProfilePictureBytes"] = 0;
if (ProfilePictureFileUploader.HasFile == true)
{

byte[] br = File.ReadAllBytes("D:/Projects/SampleProject/Images/Default Image.jpg");
Session["ProfilePictureBytes"] = br;

}
else
{

System.IO.Stream fs = ProfilePictureFileUploader.PostedFile.InputStream;
System.IO.BinaryReader br = new System.IO.BinaryReader(fs);
byte[] profilepictureBytes = br.ReadBytes((Int32)fs.Length);
Session["ProfilePictureBytes"] = profilepictureBytes;
}

here the ProfilePictureFileUploader is the FileUpload that i have mentioned in my aspx page.
I have applied a Session named Session["ProfilePictureBytes"] which will save the bytes of the image. [the purpose of Session["ProfilePictureBytes"] is to retrieve these bytes in some other screen.]
here if the user upload a picture,then the tool will assume the else part and it will do the further things....but if the user is not uploading any picture,then the picture will be taken from my local D drive and it's saving the picture safely in the datebase...but if the user upload any picture then the else part is showing some error like "NullReferenceException was unhandled by user code" ..any idea for this problem friends? ....

Very sorry for my immature english and thanks in advance :)