无法访问已释放的对象

Imports System.Windows.Forms
Public Class Form1
    Dim sform As New Form2
    Dim ssform As New Form3

    Private Sub TreeView1_AfterSelect(ByVal sender As System.Object, ByVal e As System.Windows.Forms.TreeViewEventArgs) Handles TreeView1.AfterSelect
        If TreeView1.SelectedNode.Text = "1" Then
            sform.MdiParent = Me
            sform.LayoutMdi(MdiLayout.Cascade)
            sform.Show()
        ElseIf TreeView1.SelectedNode.Text = "2" Then
            ssform.MdiParent = Me
            ssform.LayoutMdi(MdiLayout.Cascade)
            ssform.Show()

        Else : MsgBox("as")

        End If
    End Sub

点击子窗体的关闭窗口键后再次点击节点 提示错误：无法访问释放的对象！ 求解啊
[解决办法]
sform、ssform 在它关闭时，禁止它关闭，改为隐藏窗口。

[解决办法]
If sform.IsDisposed Then
sform = new Form2
End If
sform.MdiParent = Me
sform.LayoutMdi(MdiLayout.Cascade)
sform.Show()