将MS Access 2013数据库与vb.net应用程序连接时出错

如何解决将MS Access 2013数据库与vb.net应用程序连接时出错

我无法连接我的mdb数据库。我试图将一些数据从表单添加到数据库中，但是当我运行该应用程序并单击添加按钮时，它显示以下错误：- “ System.NullReferenceException：'对象引用未设置为对象的实例。'”“ cmd没什么。”这是我的代码：-

Public Class addForm
Dim cnn As OleDb.OleDbConnection
Dim cmd As OleDb.OleDbCommand
Private Sub addForm_Load(sender As Object,e As EventArgs) Handles MyBase.Load

End Sub

Private Sub btnAdd_Click(sender As Object,e As EventArgs) Handles btnAdd.Click

    Try
        cnn = New OleDb.OleDbConnection
        cnn.ConnectionString = "Provider=Microsoft.ACE.OLEDB.12.0; Data Source=C:\Users\krish\source\repos\DDC Library Management System\DDC Library Management System\bin\Debug\data.accdb"
        cmd.Connection = cnn
        cnn.open()
        cmd.CommandText = "INSERT INTO Books(BookType,BookCode,BookName,Publication,Price,Author,dt)VALUES(@bookType,@bookCode,@bookName,@publication,@price,@author,@dt)"
        cmd.Parameters.AddWithValue("@bookType",bookType.SelectedItem)
        cmd.Parameters.AddWithValue("@bookCode",bookCode.Text)
        cmd.Parameters.AddWithValue("@bookName",bookName.Text)
        cmd.Parameters.AddWithValue("@publication",txtPublication.Text)
        cmd.Parameters.AddWithValue("@price",txtPrice.Text)
        cmd.Parameters.AddWithValue("@author",txtAuthor.Text)
        cmd.Parameters.AddWithValue("@dt",dtDate.Value)
        cmd.ExecuteNonQuery()
        cnn.Close()
        MessageBox.Show("Data Added")
    Catch ex As Exception
        MessageBox.Show(ex.Message)
    End Try
End Sub End Class

“ addForm”是我当前表单的名称。

This is the error it is showing as soon as i click on ADD Button