✅ Strange error when trying to retrun a class type variable through a public method

namespace InventoryMaintenance
{
    public partial class frmNewInventoryItem : Form
    {
        InventoryItem invItem = null;

        public frmNewInventoryItem()
        {
            InitializeComponent();
        }

        public InventoryItem GetNewItem()
        {
            this.ShowDialog();

            return invItem; 
        }
        private void BtnSave_Click(object sender, EventArgs e)
        {

        }

        private void BtnCancel_Click(object sender, EventArgs e)
        {

        }
    }
}