Object reference not set to an instance of an object

Private Sub txtSpelling_MouseDown(ByVal sender As System.Object, ByVal e As System.Windows.Forms.MouseEventArgs) Handles txtActivityDesc.MouseDown

        Try
            Dim txt As TextBox = DirectCast(sender, TextBox)

            If Not txt.Focused Then
                txt.Focus()
                SendKeys.Send("{HOME}")
            End If

            If e.Button = System.Windows.Forms.MouseButtons.Right Then
                Me.spellingError = Me.UltraSpellChecker1.GetErrorAtPoint(txt, New Point(e.X, e.Y))
                ' If there is no spelling error at the designated coordinates, clear the context menu and cancel.
                If spellingError Is Nothing Then
                    Me.CMenuSpell.Items.Clear()

                    Me.CMenuSpell.Items.Add(New ToolStripMenuItem("Spell Check", Nothing, New System.EventHandler(AddressOf mSpellChk_Click)))

                    Me.CMenuSpell.Items.Add(New ToolStripSeparator)
                    Me.CMenuSpell.Items.Add(New ToolStripMenuItem("Lower Case (All)", Nothing, New System.EventHandler(AddressOf mLower_Click)))
                    Me.CMenuSpell.Items.Add(New ToolStripMenuItem("Upper Case (All)", Nothing, New System.EventHandler(AddressOf mUpper_Click)))

                    Return
                Else
                    Me.PopulateCMenuonErr(spellingError)
                End If

            End If
        Catch ex As Exception
            MessageBox.Show(ex.Message & vbCrLf & vbCrLf & "[ Error Code ]    " & GetCurrentMethod.Name & " ()", MsgHeader, MessageBoxButtons.OK, MessageBoxIcon.Information)
        End Try
    End Sub