Initial state of VB .net checkbox to toggle ArcMap layer visibility

I have a checkbox control as part of an ArcMap add-in with a dockable window in VB .net that turns a particular layer’s visibility on and off in ArcMap. The initial state is set to “unchecked”. But, if I load the tool into ArcMap and use it when that layer is already visible, the checkbox works in reverse (checked when the layer is off and unchecked when the layer is on). Is there any way I can check the visibility of the layer when the tool is first loaded and then set the state of the checkbox?

I tried putting some code in Public Sub New() function for my dockable window, but this crashed ArcMap. Here is what I had (with 3 dots representing other code). My checkbox is named chkBasemap. In this simple example, there are always only 2 layers in my map. I just assume the basemap layer I want to toggle the visibility of is always the second layer. The first layer is a polygon layer.

Imports ESRI.ArcGIS.ArcMapUI
Imports ESRI.ArcGIS.Carto
Imports ESRI.ArcGIS.CartoUI
Imports ESRI.ArcGIS.Display
Imports ESRI.ArcGIS.Framework
Imports ESRI.ArcGIS.Geodatabase
Imports ESRI.ArcGIS.Geometry
Imports ESRI.ArcGIS.esriSystem

Public Class DockWinClimateTools

  Public Sub New(ByVal hook As Object)
        InitializeComponent()
        AddItems()
        CheckStates()
        Me.Hook = hook
  End Sub

  Private m_hook As Object
  Public Property Hook() As Object
    Get
      Return m_hook
    End Get
    Set(ByVal value As Object)
      m_hook = value
    End Set
  End Property

  Public Class AddinImpl
    Inherits ESRI.ArcGIS.Desktop.AddIns.DockableWindow

    Private m_windowUI As DockWinClimateTools

    Protected Overrides Function OnCreateChild() As System.IntPtr
      m_windowUI = New DockWinClimateTools(Me.Hook)
      Return m_windowUI.Handle
    End Function

    Protected Overrides Sub Dispose(ByVal Param As Boolean)
      If m_windowUI IsNot Nothing Then
        m_windowUI.Dispose(Param)
      End If
    End Sub
  End Class

  Private Sub chkBasemap_CheckedChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles chkBasemap.CheckedChanged
      Dim pMxDoc As IMxDocument = My.ArcMap.Application.Document
      Dim pMap As IMap = pMxDoc.FocusMap
      Dim pLayer As ILayer = pMap.Layer(1)

      If pLayer.Visible Then
          pLayer.Visible = False
      Else
          pLayer.Visible = True
      End If

      pMxDoc.ActiveView.ContentsChanged()
      pMxDoc.ActiveView.Refresh()
  End Sub

  Public Sub CheckStates()
        Dim pMxDoc As IMxDocument = My.ArcMap.Application.Document
        Dim pMap As IMap = pMxDoc.FocusMap
        Dim baseLayer As ILayer = pMap.Layer(1)
        If baseLayer.Visible Then
            chkBasemap.Checked = True
        Else
            chkBasemap.Checked = False
        End If
   End Sub
End Class