Tagged Values reference problem (#1603)

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

at SoftwareIdeasModeler.Core.DiagramItems.Fields.TaggedValueField.GetHashCode()

at SoftwareIdeasModeler.Core.DiagramItems.Fields.ConcreteField.GetHashCode()

at System.Collections.Generic.ObjectEqualityComparer`1.GetHashCode(T obj)

at System.Collections.Generic.HashSet`1.InternalGetHashCode(T item)

at System.Collections.Generic.HashSet`1.Contains(T item)

at SoftwareIdeasModeler.Core.Diagramming.Canvas.CanvasProcessor.ProcessSelection(Point location, PenModifiers modifiers)

at SoftwareIdeasModeler.Core.Diagramming.Canvas.CanvasProcessor.MouseDown(Point location, Point screenLocation, PenModifiers modifiers)

at SoftwareIdeasModeler.SimDiagramControl.canvas_MouseDown(Object sender, MouseEventArgs e)

at System.Windows.Forms.Control.OnMouseDown(MouseEventArgs e)

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

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

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

OF=SoftwareIdeasModeler.MainForm

LastCmd=SoftwareIdeas.Common.Commands.CombinedCommand