Error opening project file

System.NullReferenceException: Object reference not set to an instance of an object.

at SoftwareIdeasModeler.Core.DiagramItems.Concrete.ConcreteRelation.BindToEntities(Diagram diagram)

at SoftwareIdeasModeler.Core.Projecting.Project.BindOtherConcreteItems(Folder folder)

at SoftwareIdeasModeler.Core.Projecting.Project.BindEntities()

at SoftwareIdeasModeler.ProjectXml.SimXmlReader.Load()

at SoftwareIdeasModeler.ProjectHelper.LoadProject(String localFileName, Boolean showReadOnlyWarning, Boolean checkFileVersion, Boolean processCustomDiagrams, Boolean showErrorMessages, Boolean addToHistory)

at SoftwareIdeasModeler.ProjectHelper.LoadProject(String filename, String repositoryPath, Boolean showReadOnlyWarning)