<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<hibernate-mapping default-access="field.pascalcase-m-underscore" xmlns="urn:nhibernate-mapping-2.0">
  <class name="NHibernate.Bugs.Base, NHibernate.Bugs" table="Base" proxy="NHibernate.Bugs.Base, NHibernate.Bugs">
    <id name="ObjectId" type="Int32" unsaved-value="-1">
      <generator class="native" />
    </id>
    <version column="VersionCounter" name="VersionCounter" type="Int32" />
  </class>
  <joined-subclass name="NHibernate.Bugs.Parent, NHibernate.Bugs" table="Parent" proxy="NHibernate.Bugs.Parent, NHibernate.Bugs" extends="NHibernate.Bugs.Base, NHibernate.Bugs">
    <key column="ObjectId" />
    <set name="Childs" lazy="true" inverse="true">
      <key column="Owner" />
      <one-to-many class="NHibernate.Bugs.Child, NHibernate.Bugs" />
    </set>
  </joined-subclass>
  <class name="NHibernate.Bugs.Child, NHibernate.Bugs" table="Child" proxy="NHibernate.Bugs.Child, NHibernate.Bugs">
    <id name="ObjectId" type="Int32" unsaved-value="-1">
      <generator class="native" />
    </id>
    <version column="VersionCounter" name="VersionCounter" type="Int32" />
    <many-to-one name="Owner" column="Owner" class="NHibernate.Bugs.Parent, NHibernate.Bugs" />
  </class>
</hibernate-mapping>

      Parent obj1 = new Parent();
      ses.Save(obj1);
      ses.Save(new Child(obj1));

The code inside class structure relevant to exception creation is:
  public class Parent : Base
  {
    private ISet m_Childs;
    public void AddChild(Child child)
    {
      // The lazy initialization of ISet is crucial. Without that, the exception does not occure
      if (m_Childs == null)
        m_Childs = new HashedSet();
      m_Childs.Add(child);
    }

    public virtual ICollection Childs
    {
      get { return m_Childs; }
    }
  }

  public class Child
  {
    public Child() : base() { }
    public Child(Parent owner) : base()
    {
      m_Owner = owner;
      m_Owner.AddChild(this);
    }
   /*
   ....
   */
  }

NHibernate.Bugs.Bug1Tester.GenerateNullReferenceException : NHibernate.ADOException : could not update: [NHibernate.Bugs.Parent#1]
  ----> System.NullReferenceException : Object reference not set to an instance of an object.
   at NHibernate.Persister.NormalizedEntityPersister.Update(Object id, Object[] fields, Boolean[] includeProperty, Boolean[] includeTable, Object oldVersion, Object obj, SqlString[] sql, ISessionImplementor session)
   at NHibernate.Persister.NormalizedEntityPersister.Update(Object id, Object[] fields, Int32[] dirtyFields, Object[] oldFields, Object oldVersion, Object obj, ISessionImplementor session)
   at NHibernate.Impl.ScheduledUpdate.Execute()
   at NHibernate.Impl.SessionImpl.Execute(IExecutable executable)
   at NHibernate.Impl.SessionImpl.ExecuteAll(IList list)
   at NHibernate.Impl.SessionImpl.Execute()
   at NHibernate.Impl.SessionImpl.Flush()
   at NHibernate.Bugs.Bug1Tester.GenerateNullReferenceException() in C:\Projects\NHibernate.Bugs\Bug1Classes.cs:line 90
--ADOException
   at NHibernate.Driver.DriverBase.GenerateCommand(Dialect dialect, SqlString sqlString)
   at NHibernate.Impl.BatcherImpl.Generate(SqlString sqlString)
   at NHibernate.Impl.BatcherImpl.PrepareCommand(SqlString sql)
   at NHibernate.Persister.NormalizedEntityPersister.Update(Object id, Object[] fields, Boolean[] includeProperty, Boolean[] includeTable, Object oldVersion, Object obj, SqlString[] sql, ISessionImplementor session)