Unable to save a Diagram .ter file. Error: 'Object reference not set to an instance of an object' (4284563)

After editing a diagram (.ter file) and when trying to Save As, the following error is shown.

'Exception: System.NullReferenceException
Message : Object reference not set to an instance of an object.
Stack Trace:
at Quest.Toad.ModelerDrawing.DrawingControls.RelationObject.WriteXml(XmlElement node)
at Quest.Toad.ModelerDrawing.DrawingControls.DiagramControl.WriteXml(XmlElement node)
at Quest.Toad.Diagramming.ErDiagramControl.DoFileSave(String file)
at Quest.Toad.Diagramming.DiagramForm.DoFileSave(String file)
at Quest.Toad.Diagramming.DiagramForm.DoFileSaveAs(String file)'
 

Unknown

WORKAROUND:
None

STATUS:
Waiting for fix in a future release of Toad for Data Analysts