Customer registration adding custom property issue

I am working on the CustomerController - Register action. I tried to add custom property to the RegisterModel.

  
public override RegisterModel PrepareRegisterModel(RegisterModel model, bool excludeProperties, string overrideCustomCustomerAttributesXml = "", bool setDefaultValues = false)
        {            
            if (!model.CustomProperties.ContainsKey("MyCustomProperty"))
            {
                model.CustomProperties.Add("MyCustomProperty"," ");
            }     

            return base.PrepareRegisterModel(model, excludeProperties, overrideCustomCustomerAttributesXml, setDefaultValues);
        }

 @Html.EditorFor(model => model.CustomProperties["MyCustomProperty"])

First Issue : if we dont give any value to the custome property, editor is not shown on the view.

model.CustomProperties.Add("MyCustomProperty",""); 

Editor is not shown on the view.

model.CustomProperties.Add("MyCustomProperty"," "); 

Editor is shown on the view.


I filled the form and also MyCustomProperty and then click save button. In the register action
ı can catch the value of my property like this

 string myCustomProperty = (model.CustomProperties["MyCustomProperty"] as string[]).FirstOrDefault(); 

Its filled OK.
On this action i made my special validations. If catch any invalid data, i throw an exception.
This my catch part

 catch (Exception ex)
                    {
                        var customerAttributesXml = ParseCustomCustomerAttributes(form);
                        model = _customerModelFactory.PrepareRegisterModel(model, true, customerAttributesXml);                        
                        ModelState.AddModelError("", ex.Message);
                        return View(model);
                    }

So i redirected to the form screen again with form datas, and i edited data and click save button again.
At this time value of MyCustomProperty is coming as null

 
 model.CustomProperties["MyCustomProperty"]  // value of this is coming null

Property is exist but value is null.


Where is the problem on my code? Or is there another way to do this.