NullReferenceException on qualified association set (#468)

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

at SoftwareIdeasModeler.Core.Commands.ChangeQualifiersCommand.SpecificExecute()

at SoftwareIdeasModeler.Core.Commands.BaseCommands.ChangeTrackedCommand.Execute()

at SoftwareIdeasModeler.Core.Commands.CombinedCommand.Execute()

at SoftwareIdeasModeler.Core.Commands.CombinedCommand.Execute()

at SoftwareIdeasModeler.Core.Projecting.CommandStack.AddCommand(Command command, Boolean execute)

at SoftwareIdeasModeler.Core.Projecting.CommandStack.Execute(Command command)

at SoftwareIdeasModeler.Core.Projecting.Project.Execute(Command cmd)

at SoftwareIdeasModeler.Dialogs.Properties.ItemPropertiesForm.okButton_Click(Object sender, EventArgs e)

at System.Windows.Forms.Control.OnClick(EventArgs e)

at System.Windows.Forms.Button.OnClick(EventArgs e)

at System.Windows.Forms.Button.OnMouseUp(MouseEventArgs mevent)

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

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

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

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

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

OF=SoftwareIdeasModeler.MainForm

LastCmd=SoftwareIdeasModeler.Core.Commands.AutoSizeCommand

OF=DockablePanel.Controls.DockablePanel.FloatingForm

OF=SoftwareIdeasModeler.Forms.StartForm

OF=SoftwareIdeasModeler.Forms.ProjectForm

OF=SoftwareIdeasModeler.Forms.EditorForm

OF=SoftwareIdeasModeler.Dialogs.Properties.ItemPropertiesForm