error creating window handle

I've tried calling an mdi child from an mdi child form.

Here is the code:

Code:

---

Dim afrmPrintTransaction as frmPrintTransaction = frmPrintTransaction.Instance
afrmPrintTransaction.connectionString=myconnString 'write only property use to pass the connection string
conn.close()
With afrmPrintTransaction
    .MdiParent=frmMainForm
    .Show()
    .Focus 'the error appears in here
End With

---

but when i tried to run it. I get the error "error creating window handle"

I've also tried to putting the code from the mdi parent and call it through the mdi child. but instead i get the error "object reference not set to an instance of an object"