DataGridViewComboBoxCell创建空引用异常

我是Stack Overflow的新手，所以请原谅任何不恰当的形式/礼仪。谢谢!

编辑:我想我的问题不是关于如何修复NullReference异常，而是如何处理它不正确地"退出"组合框当我点击它的外面。

我有一个问题与我的DataGridViewComboBoxCell设置，我正在使用。首先，我有一个包含3列的datagridview，在用户启用编辑后，用DataGridViewComboBoxCell填充单元格。在每一行中，这3个单元格依赖于前一个单元格中的选定项(第一个ComboBoxCell除外)。我遇到的问题是，如果我点击第一个ComboBox，我让它显示下拉列表，但我实际上没有选择任何东西，我移动到下一个ComboBoxCell并尝试点击它以查看其项目列表，它会停止程序并为"NullReference异常未处理"创建一个错误。此异常出现在应用程序的"static void Main()"下。运行(新MainForm ());

处理更改ComboBox选定索引的代码，该索引应该自动填充其他ComboBox。

private void LoadRules_EditingControlShowing(object sender, DataGridViewEditingControlShowingEventArgs e)
    {
        DataGridViewComboBoxEditingControl cbx = e.Control as DataGridViewComboBoxEditingControl;
        if (cbx != null)
        {
            if (this.LoadRulesDataGridView.Columns[LoadRulesDataGridView.CurrentCell.ColumnIndex].Name.Equals("OEM"))
            {
                ComboBox cmbprocess = e.Control as ComboBox;

                cmbprocess.SelectedIndexChanged += new EventHandler(OEMBox_SelectedIndexChanged);
                cmbprocess.SelectedIndexChanged += new EventHandler(ModelBox_SelectedIndexChanged);
            }
        }
    }
    private void OEMBox_SelectedIndexChanged(object sender, EventArgs e)
    {
        NetMonDB.DBManager dbConn = new NetMonDB.DBManager(ConnStr, this.LogWarning, this.LogError);
        ComboBox cmbprocess = (ComboBox)sender;
        int row = this.LoadRulesDataGridView.CurrentCell.RowIndex;
        string OEM = cmbprocess.SelectedItem.ToString();
        this.RulesGridModels(row, cmbprocess, dbConn, OEM);//this method gets the required info from the database and loads it into the ComboBox
        cmbprocess.SelectedIndexChanged -= new EventHandler(OEMBox_SelectedIndexChanged);
    }
    private void ModelBox_SelectedIndexChanged(object sender, EventArgs e)
    {
        NetMonDB.DBManager dbConn = new NetMonDB.DBManager(ConnStr, this.LogWarning, this.LogError);
        ComboBox cmbprocess = (ComboBox)sender;
        int row = this.LoadRulesDataGridView.CurrentCell.RowIndex;
        string Model = cmbprocess.SelectedItem.ToString();
        string OEM = cmbprocess.SelectedItem.ToString();
        this.RulesGridOSVersions(row, cmbprocess, dbConn, OEM, Model);//this method gets the required info from the database and loads it into the ComboBox
        cmbprocess.SelectedIndexChanged -= new EventHandler(ModelBox_SelectedIndexChanged);
    }

更新组合框的方法。

private void RulesGridModels(int r, ComboBox comboBox, NetMonDB.DBManager dbConn, string rowOEM)
    {
        //MessageBox.Show(this.LoadRulesDataGridView.Rows[0].Cells[4].Value.ToString());
        DataGridViewComboBoxCell cbo = new DataGridViewComboBoxCell();
        for (int i = 0; i < comboBox.Items.Count; i++)
        {
            cbo.Items.AddRange(comboBox.Items[i]);
        }
        try
        {
            cbo.Items.Clear();
            cbo.Items.AddRange("");
            if (this.LoadRulesDataGridView.Rows[r].Cells[4].Value == null)
                this.LoadRulesDataGridView.Rows[r].Cells[4].Value = "";
            NetMonDB.Phone OEMPhone = dbConn.getOEMId(rowOEM);
            foreach (NetMonDB.Phone phone in dbConn.getModel(OEMPhone))
            {
                cbo.Items.Add(phone.Model);
            }
            this.LoadRulesDataGridView.Rows[r].Cells[5] = cbo;
        }
        catch (Exception e)
        {
            LogError("RulesGridModels", e.ToString());
        }
    }
    private void RulesGridOSVersions(int r, ComboBox comboBox, NetMonDB.DBManager dbConn, string rowOEM, string rowModel)
    {
        DataGridViewComboBoxCell cbo = new DataGridViewComboBoxCell();
        for (int i = 0; i < comboBox.Items.Count; i++)
        {
            cbo.Items.AddRange(comboBox.Items[i]);
        }
        try
        {
            cbo.Items.Clear();
            cbo.Items.AddRange("");
            NetMonDB.Phone CurrentPhone = dbConn.getOEMId(rowOEM);
            CurrentPhone.Model = rowModel;
            foreach (NetMonDB.Phone phone in dbConn.getOSVersion(CurrentPhone))
            {
                cbo.Items.Add(phone.OSVersion);
            }
            this.LoadRulesDataGridView.Rows[r].Cells[6] = cbo;
        }
        catch (Exception e)
        {
            LogError("RulesGridOSVersions", e.ToString());
        }
    }

异常被捕获的位置。

static void Main()
    {
        Application.EnableVisualStyles();
        Application.SetCompatibleTextRenderingDefault(false);
        Application.Run(new MainForm());//Crashes at this Point
    }