Custom Server Controls :: Custom Control - Default Value Of Attributes?

I have created a custom control and I intended to assign some attributes which is not necessary to implament.

I also assign default value for them but I am getting System.NullReferenceException error.

Here is my CC's attibute code;

public class ApplicationModule2010 : CompositeControl
{
[DefaultValue(true)]
public bool AppFormOnLine { get { return (bool)ViewState["AppFormOnLine"]; } set { ViewState["AppFormOnLine"] = value; } }
...

also ı have written the below code for another attribute but its default value doesn't work as well;

[DefaultValue("Application is Offline ! Define an offline Text insted of this text !<br/><br/>TugberkUgurlu.Com")]
public string OfflineText { get { return (string)ViewState["OfflineText"]; } set { ViewState["OfflineText"] = value; } }

What am I missing here ?