Creating an instance of an object

Question

I've been trying to use this code but the null exception reference occurs
this is the code:

        public string _textBoxProperty
        {
 
            set
            {string str = webBrowser1.Document.GetElementById("Age").InnerText;
              str= value; }
            get { return webBrowser1.Document.GetElementById("Age").InnerText; }
        }

its function is a bit complicated so I avoided telling it,
I would be obliged if someone helps me out.