We experienced a crash in our program that we are now unable to reproduce. I am trying to put in some code to prevent it from happening again but I am confused over the stack trace.

System.NullReferenceException: Object reference not set to an instance of an object.
   at System.Object.GetType()
   at Project.ViewModel.MainVM.<CreateCommands>b__8(Object a)
   at System.Windows.Controls.Button.OnClick()

-- I have cut down the stack trace as it just goes into a load of system code which is just to do with the button being clicked. --

I have managed to deduce that it is pointing to my anonymous delegate on line 8 of my CreateCommands method.

        this.sectionCommand = new DelegateCommand(a =>
        {
            this.OnSectionParameterChanged((Sections)a);
        }, p => this.IsSectionCommandExecutable);

I have seen a similar post on here but the OP was calling GetType explicitly. I am assuming that the cast calls get type, but without being able to reproduce the issue I cannot see what is null.

So my question is: For this stack trace to cause a null reference, is the 'a' variable the null object? (so I would write something like)

            if (a != null)
            {
                this.OnSectionParameterChanged((Sections)a);
            } 

or is the cast from 'a' to 'sections' causing a null object? (so I should write something like)

            if (a is Sections)
            {
                this.OnSectionParameterChanged((Sections)a);
            } 

As requested here is OnSectionParameterChanged

    private void OnSectionParameterChanged(Sections parameter)
    {
        this.SelectedSection = parameter;

        this.RaisePropertyChanged(() => this.SelectedSection);

        this.LoadSettingsPanel();
    }

further to that it calls a LoadSettingsPanel

    private void LoadSettingsPanel()
    {
        if (sectionVMs == null)
            return;

        // Get section
        SectionViewModel = sectionVMs.SingleOrDefault(s.SectionName == SelectedSection);

        this.IsSelectedSectionEnabled = this.Config.GetIsSectionEnabled(this.SelectedSection);

        this.RaisePropertyChanged(() => this.IsSelectedSectionEnabled);

        // Set advanced
        AdvancedViewModel = this.SectionViewModel;

        if (AdvancedViewModel != null)
            HasAdvanced = AdvancedViewModel.HasAdvanced;
    }