NullReferenceException when closing our application

We tripped over this exception when trying to close down our application:

System.NullReferenceException: Object reference not set to an instance of an object.
at ActiproSoftware.SyntaxEditor.Document.get_Characters(Int32 index)
at ActiproSoftware.SyntaxEditor.DisplayLineCollection.a(Int32 A_0)
at ActiproSoftware.SyntaxEditor.DisplayLine.get_Length()
at
ActiproSoftware.SyntaxEditor.EditorView.PositionToOffset(EditPosition
position)
at ActiproSoftware.SyntaxEditor.EditorView.GetCursor(Point point)
at ActiproSoftware.WinUICore.UIControl.OnMouseMove(MouseEventArgs e)
at
ActiproSoftware.SyntaxEditor.SyntaxEditor.OnMouseMove(MouseEventArgs e)
at System.Windows.Forms.Control.WmMouseMove(Message& m)
at System.Windows.Forms.Control.WndProc(Message& m)
at System.Windows.Forms.ScrollableControl.WndProc(Message& m)
at ActiproSoftware.SyntaxEditor.SyntaxEditor.WndProc(Message& m)
at
System.Windows.Forms.Control.ControlNativeWindow.OnMessage(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)

I don't believe there were any syntax errors or Intellisense popups of any kind showing. Not sure how to reproduce it. Sorry :(