Can't open project file

I am getting the following error on opening a project:

System.NullReferenceException: Referência de objeto não definida para uma instância de um objeto.

em SoftwareIdeasModeler.Core.Projecting.Folder.LoadItemsFromXElement(SimXmlReader reader, Folder folder, XElement root)

em SoftwareIdeasModeler.Core.Projecting.Folder.FromXElement(SimXmlReader reader, Project project, XElement elem)

em SoftwareIdeasModeler.Core.Projecting.Folder.FromXElement(SimXmlReader reader, Project project, XElement elem)

em SoftwareIdeasModeler.Core.Projecting.Folder.FromXElement(SimXmlReader reader, Project project, XElement elem)

em SoftwareIdeasModeler.ProjectXml.SimXmlReader.LoadFolders(XElement root)

em SoftwareIdeasModeler.ProjectXml.SimXmlReader.Load()

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

em SoftwareIdeasModeler.ProjectHelper.LoadProject(String filename, String repositoryPath)

(error text in english: "Undefined project reference for an object instance" or something like that)

I tried to look on XML project file, but have no idea what could be wrong with it.