error opening diagram (#137)

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

em SoftwareIdeasModeler.Core.DiagramItems.HierarchicalTaskAnalysis.Task.GetNumberPrefix()

em SoftwareIdeasModeler.Core.DiagramItems.HierarchicalTaskAnalysis.Task.GetFullNumber()

em SoftwareIdeasModeler.Core.DiagramItems.HierarchicalTaskAnalysis.ConcreteTask.DrawElement(DiagramDrawer drawer, DrawingParameters param)

em SoftwareIdeasModeler.Core.Projecting.ConcreteDiagramItem.Draw(DiagramDrawer drawer, DrawingParameters param)

em SoftwareIdeasModeler.Visualisation.DiagramDrawer.Draw(DiagramLayer layer, ConcreteDiagramItem item)

em SoftwareIdeasModeler.Visualisation.DiagramDrawer.Draw(DiagramLayer layer, Rectangle clipArea)

em SoftwareIdeasModeler.Visualisation.DiagramDrawer.Draw(Diagram diagram, Rectangle drawBounds)

em SoftwareIdeasModeler.Core.Exporting.Images.BitmapDiagramExporter.GetThumbnail(Diagram diagram, Size size, Boolean optimized)

em SoftwareIdeasModeler.MdiChildrenForms.EditorForm.GetThumbnail()

em SoftwareIdeasModeler.MainForm.UpdateWindowsTabSwitch()

em SoftwareIdeasModeler.MainForm.MainForm_MdiChildActivate(Object sender, EventArgs e)

em System.Windows.Forms.Form.OnMdiChildActivate(EventArgs e)

em SoftwareIdeasModeler.MainForm.OnMdiChildActivate(EventArgs e)

em System.Windows.Forms.Form.WmMdiActivate(Message& m)

em System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)

em System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)