Add to list mvc - asp.net-mvc

Question

I'm trying to add string data to list, but have null reference exception and don't have any idea how to fix this.
Here home controller:
public ActionResult Index()
{
return View();
}
[HttpPost]
public ActionResult Index(ChatUser model)
{
if (ModelState.IsValid)
{
**model.chatGroup.Add(model.inputGroups);** - here excepton
}
return View(model);
}
And Index.cshtml:
#model test.Models.ChatUser
#{
ViewBag.Title = "Test";
}
<h2>#ViewBag.Title.</h2>
#using (Html.BeginForm("Index", "Home", FormMethod.Post, new { #class = "form-horizontal", role = "form" }))
{
<div class="form-group">
#Html.LabelFor(m => m.inputGroups, new { #class = "col-md-2 control-label" })
<div class="col-md-10">
#Html.TextBoxFor(m => m.inputGroups, new { #class = "form-control" })
<input type="submit" value="GO" class="btn btn-default" />
</div>
</div>
}
#section Scripts {
#Scripts.Render("~/bundles/jqueryval")
}

Answer 1

Your form does not include any controls for properties of chatGroup so its null when you post back. You need to either initialize the property in a parameterless constructor
public class ChatUser
{
public ChatUser()
{
chatGroup = new List<string>(); // initialize collection
}
....
public List<string> chatGroup { get; set; }
}
or initialize it in the POST method
[HttpPost]
public ActionResult Index(ChatUser model)
{
if (ModelState.IsValid)
{
model.chatGroup = new List<string>(); // initialize collection
model.chatGroup.Add(model.inputGroups);
}
return View(model);
}