XMI export/import

I tried to export to XMI and import to an empty project, got the following error:

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

at SoftwareIdeasModeler.Core.Xmi.NewXmiImporter.ProcessGeneralization(Model model, ModelElement item, Generalization generalization)

at SoftwareIdeasModeler.Core.Xmi.NewXmiImporter.ProcessElement(Model model, PackagedElement element)

at SoftwareIdeasModeler.Core.Xmi.NewXmiImporter.ProcessElement(Model model, PackagedElement element)

at SoftwareIdeasModeler.Core.Xmi.NewXmiImporter.ProcessModel(XmiUmlModel model)

at SoftwareIdeasModeler.Core.Xmi.NewXmiImporter.ProcessFile(XmiFile file)

at SoftwareIdeasModeler.Core.Xmi.NewXmiImporter.ImportFile(String fileName)

at SoftwareIdeasModeler.Core.Xmi.XmiImporter.ImportFile(String fileName)

at SoftwareIdeasModeler.MainForm.importXMIToolStripMenuItem_Click(Object sender, EventArgs e)

at System.Windows.Forms.ToolStripItem.RaiseEvent(Object key, EventArgs e)

at System.Windows.Forms.ToolStripMenuItem.OnClick(EventArgs e)

at System.Windows.Forms.ToolStripItem.HandleClick(EventArgs e)

at System.Windows.Forms.ToolStripItem.HandleMouseUp(MouseEventArgs e)

at System.Windows.Forms.ToolStrip.OnMouseUp(MouseEventArgs mea)

at System.Windows.Forms.ToolStripDropDown.OnMouseUp(MouseEventArgs mea)

at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)

at System.Windows.Forms.Control.WndProc(Message& m)

at System.Windows.Forms.ToolStrip.WndProc(Message& m)

at System.Windows.Forms.ToolStripDropDown.WndProc(Message& m)

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

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

But I don't really understand how XMI export/import works? Should I be able to export and import a specific diagram or is it always the entire project or what. Also need to mention that I solely uses the sysML diagrams currently.