Assimp: Unable to export parts of a weighted model

[Nadevan]

Program: SAMDL
Build Date: 08/19/2021 17:36:58
OS Version: Microsoft Windows NT 10.0.19042.0
Log:
System.NullReferenceException: Object reference not set to an instance of an object.
at SonicRetro.SAModel.Vertex.op_Addition(Vertex a, Vertex b) in D:\Programs\sa_tools\SAModel\Vertex.cs:line 202
at SonicRetro.SAModel.SAEditorCommon.Import.AssimpStuff.AssimpExportWeighted(NJS_OBJECT obj, Scene scene, Matrix parentMatrix, String[] texInfo, Node parent, Int32& mdlindex) in D:\Programs\sa_tools\SAEditorCommon\Import\AssimpStuff.cs:line 160
at SonicRetro.SAModel.SAEditorCommon.Import.AssimpStuff.AssimpExportWeighted(NJS_OBJECT obj, Scene scene, Matrix parentMatrix, String[] texInfo, Node parent) in D:\Programs\sa_tools\SAEditorCommon\Import\AssimpStuff.cs:line 60
at SonicRetro.SAModel.SAEditorCommon.Import.AssimpStuff.AssimpExport(NJS_OBJECT obj, Scene scene, Matrix parentMatrix, String[] texInfo, Node parent) in D:\Programs\sa_tools\SAEditorCommon\Import\AssimpStuff.cs:line 48
at SonicRetro.SAModel.SAMDL.MainForm.ExportModel_Assimp(String filename, Boolean selected) in D:\Programs\sa_tools\SAMDL\MainForm.cs:line 2390
at SonicRetro.SAModel.SAMDL.MainForm.exportContextMenuItem_Click(Object sender, EventArgs e) in D:\Programs\sa_tools\SAMDL\MainForm.cs:line 3354
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.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)