错误 - 未将对象引用设置为对象的实例

Question

我正在创建一个预订学校表演门票的计划。在这部分代码中，我试图显示所有演出的可用日期，这些日期来自数据库。然后根据选择的日期，我改变座位计划的颜色以指示座位是否被拍摄。

 Private Sub Seating_Plan_Load(sender As Object, e As EventArgs) Handles MyBase.Load
    ds.Clear()
    con.Open()
    sqlstatement = "SELECT ShowDate FROM AvailableDates"
    da = New OleDb.OleDbDataAdapter(sqlstatement, con)
    da.Fill(ds, "dates")
    lbxDates.DataSource = ds.Tables("Dates")
    lbxDates.DisplayMember = "ShowDate"
    lbxDates.ValueMember = "ShowDate"
    con.Close()
End Sub


Private Sub lbxDates_SelectedValueChanged(sender As Object, e As EventArgs) Handles lbxDates.SelectedValueChanged


    Dim oDataRowView As DataRowView = CType(Me.lbxDates.SelectedItem, DataRowView)

    If Not IsDBNull(oDataRowView) Then
        oDataRowView = CType(Me.lbxDates.SelectedItem, DataRowView)
    End If
    sSelectedAssetType = oDataRowView("ShowDate").ToString



    For Each btn As Control In Me.Controls
        If checkSeats(btn.Name()) = "True" Then
            btn.BackColor = Color.LightGreen
        Else
            btn.BackColor = Color.Coral
        End If

        If btn.Name() = "lblStage" Then
            btn.BackColor = Color.Transparent
        ElseIf btn.Name() = "btnClose" Then
            btn.BackColor = Color.Transparent
        ElseIf btn.Name() = "btnBookBack" Then
            btn.BackColor = Color.Transparent
        ElseIf btn.Name() = "pbxBob" Then
            btn.BackColor = Color.Transparent
        ElseIf btn.Name() = "lblTitle" Then
            btn.BackColor = Color.Transparent
        ElseIf btn.Name() = "Button1" Then
            btn.BackColor = Color.Transparent
        ElseIf btn.Name() = "lblTaken" Then
            btn.BackColor = Color.LightGreen
        ElseIf btn.Name() = "lblTaken2" Then
            btn.BackColor = Color.Coral
        ElseIf btn.Name() = "lblConcertDate" Then
            btn.BackColor = Color.Transparent
        ElseIf btn.Name() = "txtDate" Then
            btn.BackColor = Color.White
        ElseIf btn.Name() = "lblShowDate" Then
            btn.BackColor = Color.Transparent
        ElseIf btn.Name() = "lbxDates" Then
            btn.BackColor = Color.White
        End If

    Next
End Sub

Public Function checkSeats(ByVal seatRef As String)

    con.Close()
    ds.Clear()
    con.ConnectionString = connString
    con.Open()                                          'Open connection to the database
    sqlstatement = "SELECT * FROM " & "Seats" & sSelectedAssetType & " WHERE [Available] = True AND [Seat_Location] ='" & seatRef & "'"
    da.Fill(ds, "Seats")                           'Fill the data adapter
    con.Close()
    Dim recordCount, x As Short
    recordCount = 0
    x = 0
    recordCount = ds.Tables("seats").Rows.Count
    If recordCount = 0 Then
        Return False
    Else
        Return True
    End If
End Function

连接数据库的所有数据都在一个模块中，并且工作正常，我得到错误的行是这个：

sSelectedAssetType = oDataRowView("ShowDate").ToString