Exception existing diagrams (#350)

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

at SoftwareIdeasModeler.Controls.FastTree.FastTree.FastTree_DoubleClick(Object sender, EventArgs 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.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)