I have an mdi parent and 2 child forms. Both are opened. Now When I try to pass a value, it gives me nullexception value.

Heres how I created MDI child forms:

Dim _a As New a
        _a.MdiParent = Me

Dim _b As New b
        _b.MdiParent = Me

When both are opened with "a" having some text in textbox1 and "b" having empty textbox2. Now, I tried to get the value for "b" textbox from "a" as textbox2.text=a.textbox1.text. It gives me error. No value can be passed between child forms? How can I share the form events now?