mdichild form not showing

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VB

MDI

Hi, just would like to ask if someone encountered the same problem with mine. I have created a vb.net application using 5 forms which form1 is the parent form and the other 4 is the child. I already included all the codes necessary to handle the child forms and it is doing fine but when i created a menu to control the child forms the problem is when i close one of the child form then click the menu to activate the form again, there is no result. The menu is only good at closing but not on showing.

Here is the code.

 Public mw As Form2 = New Form2
    Public calc As Form3 = New Form3
    Public curr As Form4 = New Form4
    Public res As Form5 = New Form5
 Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        Me.IsMdiContainer = True

        mw.Owner = Me
        calc.Owner = Me
        curr.Owner = Me
        res.Owner = Me

        mw.MdiParent = Me
        calc.MdiParent = Me
        curr.MdiParent = Me
        res.MdiParent = Me

        res.Show()
        curr.Show()
        calc.Show()
        mw.Show()

 Me.LayoutMdi(MdiLayout.TileVertical)
End Sub

 Private Sub SampleMenuMW_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles SampleMenuM.Click
        If mw IsNot Nothing Then
            mw.Close()
        Else
            mw.Show()
        End If
    End Sub

Thank you very much.