object reference error

[phpman]

I have this bit of code

Private Sub SpellChkMenuItem_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles SpellChkMenuItem.Click
        Dim activeRichTextBox = TryCast(Me.ActiveControl, RichTextBox)
        'clsFS.CheckSpelling(activeRichTextBox)
        'clsFS.SpellAndGrammarCheck(activeRichTextBox)
        If Len(activeRichTextBox.Text) > 0 Then
            SpellChecker.Text = activeRichTextBox.Text
            SpellChecker.SpellCheck()
        Else
            MsgBox("Text box is empty")
            End If
    End Sub

When I am in the rtb and click on spell check button it tells me the error

"Object reference not set to an instance of an object."

On this line

SpellChecker.Text = activeRichTextBox.Text

But it is suppose to be getting the focused rtb correct? So why is it not referenced