CommunitySystem.CurrentContext.DefaultSecurity.RemoveUser(IUser user);

To remove user (Just as the answer posted in this issue).

However, this works fine. But for one user it gives an error. The exception throws from the RemoveUser(IUser user).

The exception details are here as below,

System.ArgumentNullException: The type cannot be null. Parameter name: type 
at EPiServer.Data.Entity.Internal.EntityTypeResolver.GetTypeName(Type type, Boolean ignoreOverrideAttribute) 
at EPiServer.Common.ImplementationModule.HandleCommentAddUpdateRemove(IComment comment, SupportedOperations operation) 
at EPiServer.Common.Comments.CommentHandler.OnCommentUpdated(IComment comment) 
at EPiServer.Data.Providers.Internal.SqlDatabaseExecutor.<>c__DisplayClass32_0.b__0() 
at EPiServer.Data.Providers.Internal.SqlDatabaseExecutor.<>c__DisplayClass33_0`1.b__0() 
at EPiServer.Common.Comments.CommentHandler.UpdateComment(IComment comment) 
at EPiServer.Common.Data.FrameworkFactoryBase.CascadeUpdateStatus(IEntityStatus entity, ICollection`1 children) 
at EPiServer.Common.Data.CommentFactory.CascadeAuthorStatus(IAuthor author) 
at EPiServer.Common.AuthorEventHandler.Invoke(EntityEventArgs args) 
at EPiServer.Common.AuthorHandler.OnAuthorUpdated(IAuthor author) 
at EPiServer.Data.Providers.Internal.SqlDatabaseExecutor.<>c__DisplayClass32_0.b__0() 
at EPiServer.Data.Providers.Internal.SqlDatabaseExecutor.<>c__DisplayClass33_0`1.b__0() 
at EPiServer.Common.AuthorHandler.UpdateAuthor(IAuthor author) 
at EPiServer.Common.Data.FrameworkFactoryBase.CascadeUpdateStatus(IEntityStatus entity, ICollection`1 children) 
at EPiServer.Common.Security.Data.SecurityFactory.<>c__DisplayClass57_0.b__0() 
at EPiServer.Data.Providers.Internal.SqlDatabaseExecutor.<>c__DisplayClass32_0.b__0() 
at EPiServer.Data.Providers.Internal.SqlDatabaseExecutor.<>c__DisplayClass33_0`1.b__0() 
at EPiServer.Data.Providers.SqlTransientErrorsRetryPolicy.Execute[TResult](Func`1 method) 
at EPiServer.Common.Security.SecurityHandler.RemoveUser(IUser user)

I have decompiled the 'EPiServer.Common.ImplementationModule' and this is what the 'HandleCommentAddUpdateRemove(..)' has.

private static void HandleCommentAddUpdateRemove(IComment comment, SupportedOperations operation) {
    EntityReference commentedEntity = comment.CommentedEntity;
    IComment comment2 = null;
    CommentEventData commentEventData;
    if ((int) operation == 4) {
        comment2 = (IComment) comment.get_Master();
        if (comment2 == null || comment2.Status == comment.Status) {
            return;
        }
        commentedEntity = comment2.CommentedEntity;
        commentEventData = new CommentEventData(IEntityTypeResolverExtensions.GetTypeName(ServiceLocator.get_Current().GetInstance < IEntityTypeResolver > (), commentedEntity.get_Type()), commentedEntity.get_ID(), comment2.Status, -1);
        m_commentEvent.Raise(m_commentId, (object) commentEventData);
    }
    commentEventData = new CommentEventData(IEntityTypeResolverExtensions.GetTypeName(ServiceLocator.get_Current().GetInstance < IEntityTypeResolver > (), commentedEntity.get_Type()), commentedEntity.get_ID(), comment.Status, ((int) operation != 8) ? 1 : (-1));
    m_commentEvent.Raise(m_commentId, (object) commentEventData);
}

What causes this to happen ?

It seems like the below code returns 'Null'

commentedEntity.get_Type();

Any way to delete this user?

Additional info:

EPiServer.CommonFramework version=9.0.1
EPiServer.CMS.Core version=11.11.0
EPiServer.Community version=9.0.0