What am I doing wrong in my custom event in Window Forms?

Question

I'm working on a project that requires me to create items in one form, put them in a List in another and them insert them for display on a list view on a third. I have a custom event set up to add the items to their appropriate place but it doesn't work and I can't figure out why.

My UserInputForm is where the info is put in. I have a get/set method to pull the data from the fields. The declared EventHandler is in this form.

public partial class UserInputForm : Form
{
    public UserInputForm()
    {
        InitializeComponent();
    }

    MainForm main;

    public EventHandler AddNewItem;

    public Items EachItem
    {
        get
        {
            Items newItem = new Items();
            newItem.Name = nameBox.Text;
            newItem.Month = monthBox.Text;
            newItem.Day = dayBox.Value;
            newItem.Senior = seniorCheckBox.Checked;
            newItem.ImageIndex = monthBox.SelectedIndex;
            return newItem;
        }

        set
        {
            nameBox.Text = value.Name;
            monthBox.Text = value.Month;
            dayBox.Value = value.Day;
            seniorCheckBox.Checked = value.Senior;
        }
    }

    private void AddItem (object sender, EventArgs e)
    {

        if (main == null)
        {
            main = new MainForm();
        }

        AddNewItem += main.AddNewItemHandler;

        if (AddNewItem != null)
        {
            AddNewItem(this, new EventArgs());
        }

        EachItem = new Items();
    }

}

In the Main, I have the event that adds the item to the list, calls a method to display the # of items in the list, and calls a method in the ListView Form to add it there.

    public partial class MainForm : Form
{
    public MainForm()
    {
        InitializeComponent();
    }

    List<Items> inputItems = new List<Items>();

    UserInputForm newInput;

    ListView list;

      public void AddNewItemHandler(object sender, EventArgs e)
    {
        newInput = sender as UserInputForm;
        Items newItem = newInput.EachItem;
        inputItems.Add(newItem);

        if (list == null)
        {
            list = new ListView();
        }

        list.AddToListView(newItem);

        TotalInList();
    }

    private void OpenInputForm(object sender, EventArgs e)
    {
        newInput = new UserInputForm();

        if (newInput.IsDisposed == true)
        {
            newInput = new UserInputForm();
        }

        newInput.Show();

        OpenInputNum();

        newInput.FormClosed += InputFormClosed;
    }

    private void OpenListView(object sender, EventArgs e)
    {
        if (Application.OpenForms.OfType<ListView>().Count<ListView>() == 1)
        {
            Application.OpenForms.OfType<ListView>().First().Close();
            displayList.Checked = false;
        }
        else
        {
            if (list == null)
            {
                list = new ListView();
            }
            if (list.IsDisposed == true)
            {
                list = new ListView();
            }

            list.Show();

            list.Load += ListOpened;

        }
    }

 private void OpenInputNum()
    {
        string inputNum = $"{Application.OpenForms.OfType<UserInputForm>().Count<UserInputForm>()}";
        openInputNumBox.Text = inputNum;
    }

    private void ListOpened(object sender, EventArgs e)
    {
        if (list == null)
        {
            list = new ListView();
        }
        if (list.IsDisposed == true)
        {
            list = new ListView();
        }

        list.AddToNewListView(inputItems);
    }

    public void TotalInList()
    {
        string listNum = $"{inputItems.Count()}";
        itemListNumBox.Text = listNum;
    }


}

The ListView Form has a method that adds all items at once when its opened or a single item when its already open. It changes the item to a ListViewItem and adds it to the list.

    public partial class ListView : Form
{
    public ListView()
    {
        InitializeComponent();
    }

    //list for listview
    List<Items> toView;

    public void AddToNewListView(List<Items> toListView)
    {
        toView = toListView;
        ListViewItem addItem = new ListViewItem();
        foreach (Items item in toView)
        {
            addItem.Text = item.ToString();
            addItem.ImageIndex = item.ImageIndex;
            addItem.Tag = item;

            itemListView.Items.Add(addItem);
        }
    }

    public void AddToListView(Items newItem)
    {
        ListViewItem addItem = new ListViewItem();

        addItem.Text = newItem.ToString();
        addItem.ImageIndex = newItem.ImageIndex;
        addItem.Tag = newItem;

        itemListView.Items.Add(addItem);
    }
}

The problems I'm having are: Though I follow the data and though it all seems to be there when needed, my ListView will not populate and neither will the count for the number of items in the main. Is there something wrong with my code?