MDI container produce an error

Hello,
I have a problem with an application I made. I made my application in VB 2005 and it's working just fine.
In order to make it look better I decided to make a master form and make it mdi container where all the other form will be shown.
I load my first form from the MenuStrip and all the buttons are just fine. From this first form I try to load another form which will produce me a report in crystal report and I need:
1) To make the second form MDI child
2) To be aple to retreive the selected row from datagrid of the first form.
This problem was created when I make the first form an MDI container and I don't know how to fix this. Bellow you will find the code that is retreiving the selected row from the first form.
Thanks.

Private Sub PrintSelectedUserID()
 
        Dim userID As Integer
 
        If frmMain.DataGridView.SelectedRows.Count = 0 And frmMain.DataGridView.RowCount = 0 And frmMain.DataGridView.SelectedCells.Count = 0 Then
            MessageBox.Show("Update not allowed as no row/cell is selected!", ".NET Remoting Sample", MessageBoxButtons.OK, MessageBoxIcon.Exclamation)
            Exit Sub
        End If
 
        'If cell only is selected, use this to find the UserID
        If frmMain.DataGridView.SelectedRows.Count = 0 And frmMain.DataGridView.SelectedCells.Count > 0 Then
            userID = CType(frmMain.DataGridView.Rows(frmMain.DataGridView.SelectedCells(0).RowIndex).Cells("UserID").Value, Integer)
            'If full row is selected, use this to find the UserID
        ElseIf frmMain.DataGridView.SelectedRows.Count > 0 And frmMain.DataGridView.SelectedCells.Count > 0 Then
            userID = CType(frmMain.DataGridView.Rows(frmMain.DataGridView.SelectedCells(0).RowIndex).Cells("UserID").Value, Integer)
        End If

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