主题：页面集合控件问题,请教老师

如图在 DropBox控件Enter事件中写入下面的代码,在切换到第二个或第三个页面的时候出现了代码中的提示,请教老师怎么避免这样的问题?

Dim lx As String = e.Form.Controls("ComboBox2").Value
Dim zf As Boolean = e.Form.Controls("RadioButton3").Checked
Dim ywc As Boolean = e.Form.Controls("RadioButton1").Checked
Dim wwc As Boolean = e.Form.Controls("RadioButton2").Checked
If ywc = False And wwc = False And zf = False Then
    MessageBox.Show("请选择要查询的条件!")
    e.Form.Controls("RadioButton2").Select()
ElseIf lx = "" Then
    MessageBox.Show("请选择要查询的类型!")
    e.Form.Controls("ComboBox2").Select()
End If