Exception canvas (#221)

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

at SoftwareIdeasModeler.DiagramTools.BaseTools.RelationTool.MouseUp(EnhancedMouseEventArgs e)

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

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

at System.Windows.Forms.Control.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)