DNN 08.00.02 (4)

When a user trying to register to the site, I am receiving this error.
An error has occurred. Error: is currently unavailable.

I can see this error log on the event viewer.

ModuleId:-1

ModuleDefId:-1

FriendlyName:

ModuleControlSource:DesktopModules/Admin/Security/Register.ascx

AbsoluteURL:/Default.aspx

DefaultDataProvider:DotNetNuke.Data.SqlDataProvider, DotNetNuke

ExceptionGUID:042e18b6-d96b-409d-8d3a-2f62a0174626

AssemblyVersion:8.0.2

PortalId:0

UserId:-1

TabId:89

RawUrl:/Register?returnurl=%252f&popUp=true

Referrer:http://dev.************.com/Register?returnurl=%252f&popUp=true

UserAgent:Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/50.0.2661.102 Safari/537.36

ExceptionHash:E8qRyED/7/d1chORX4bMLw==

Message:Object reference not set to an instance of an object.

StackTrace:

InnerMessage:Object reference not set to an instance of an object.

InnerStackTrace:

   at DotNetNuke.Services.Social.Notifications.NotificationsController.<>c.b__10_0(RoleInfo role)
   at System.Linq.Enumerable.WhereListIterator`1.MoveNext()
   at DotNetNuke.Services.Social.Notifications.NotificationsController.SendNotification(Notification notification, Int32 portalId, IList`1 roles, IList`1 users)
   at DotNetNuke.Entities.Modules.UserModuleBase.SendAdminNotification(UserInfo newUser, PortalSettings portalSettings)
   at DotNetNuke.Entities.Modules.UserModuleBase.CompleteUserCreation(UserCreateStatus createStatus, UserInfo newUser, Boolean notify, Boolean register)
   at DotNetNuke.Modules.Admin.Users.Register.CreateUser()

Source:

FileName:

FileLineNumber:0

FileColumnNumber:0

Method:

Server Name: CLE02HCWEB01

But I can see the user is getting registered and the user is in the user table too. The user can login with the login details too. But the user doesn't know that he has completed it since it throws an error.
When the user trying to login to the site, he can login to the site. But the is not notifying the registration process completion because of this error message.

How can I fix this and why is this happening?