从DataTable获取值到CmoboBox - NullReferenceException

Question

以下两种方法几乎完全相同。第一个工作正常，但第二个，LoadScreenIDs()导致以下错误：

  

“System.NullReferenceException：对象引用未设置为   一个对象的实例。“

Public Function LoadFilmIDs()
    Dim ScreeningsFilmIDTable = ChooseTable.ds.Tables("ScreeningsFilmID")
    For Each dr As DataRow In ScreeningsFilmIDTable.Rows
        cboFilm.Items.Add(dr.Item("FILMID").ToString())
    Next
End Function

Public Function LoadScreenIDs()
    Dim ScreeningsScreenIDTable = ChooseTable.ds.Tables("ScreeningsScreenID")
    For Each dr1 As DataRow In ScreeningsScreenIDTable.Rows
        cboScreen.Items.Add(dr1.Item("SCREENID").ToString())
    Next
End Function

我看了一遍，并尝试了一切;改变方法，将所有方法都放在一个方法中，重命名对象和变量，但无济于事。

请帮忙。 ：）

更新：以下是从数据库中实际请求数据的位置，实际表中肯定有数据。

Public Function LoadScreeningsScreenID() As DataSet
    ds2 = New DataSet
    Dim sql As String
    Dim da As OracleDataAdapter
    sql = "SELECT SCREENID from SYSTEM.SCREENS"
    da = New OracleDataAdapter(sql, Connect())
    da.Fill(ds2, "ScreeningsScreenID")
    CloseDB()
    Return ds2
End Function

Public Function LoadScreeningsFilmID() As DataSet
    ds3 = New DataSet
    Dim sql As String
    Dim da As OracleDataAdapter
    sql = "SELECT FILMID from SYSTEM.FILMS"
    da = New OracleDataAdapter(sql, Connect())
    da.Fill(ds3, "ScreeningsFilmID")
    CloseDB()
    Return ds3
End Function

解

Public Function PopulateAll() As DataSet
    ds = New DataSet
    Dim sql As String
    Dim sql1 As String
    Dim sql2 As String
    Dim da As OracleDataAdapter
    sql = "SELECT * from SYSTEM.SCREENINGS"
    da = New OracleDataAdapter(sql, Connect())
    da.Fill(ds, "Screenings")
    sql1 = "SELECT FILMID from SYSTEM.FILMS"
    da = New OracleDataAdapter(sql1, Connect())
    da.Fill(ds, "ScreeningsFilmID")
    sql2 = "SELECT SCREENID from SYSTEM.SCREENS"
    da = New OracleDataAdapter(sql2, Connect())
    da.Fill(ds, "ScreeningsScreenID")
    lblStatus.Text = "Data loaded successfully"
    CloseDB()
    Return ds
End Function