Code generation for property <propertyname> failed: Property accessor on object threw ... Object reference not set ...

I have added a property to a user control.  At runtime, it functions flawlessly.  At design time, however, I am repeatedly interrupted by the error:
Code generation for property 'HideAddins' failed. Errors was: 'property accessor 'Hideaddins' on object <mycontrol> threw the following exception: 'Object reference on set to an instance of an object.''

I have code in which should absolutely prevent an 'Object reference not set' condition.  I also have code which should trap it, should it somehow occur.  I also have attributes in place to try to get the property left alone at design time.  All to no avail.

What am I doing wrong?

Thanks!

<Browsable(False)> _
    <DefaultValue(False)> _
    Public Property HideAddins() As Boolean
        Get
            Try
                If Not tcNotes Is Nothing AndAlso Not tcNotes.TabPages Is Nothing Then
                    Return Not tcNotes.TabPages.ContainsKey("tpAddIns")
                Else
                    Return False
                End If
            Catch
                Return False
            End Try
        End Get
        Set(ByVal value As Boolean)
            Try
                If Not tcNotes Is Nothing AndAlso Not tcNotes.TabPages Is Nothing Then
                    If value Then
                        'tcNotes.TabPages("tpAddIns").Hide()   '.IsAccessible = Not value
                        If tcNotes.TabPages.ContainsKey("tpAddIns") Then tcNotes.TabPages.RemoveByKey("tpAddIns")
                    Else
                        'tcNotes.TabPages("tpAddIns").Show()
                        If Not tcNotes.TabPages.ContainsKey("tpAddIns") Then tcNotes.TabPages.Add(tpAddIns)
                    End If
                End If
            Catch
            End Try
        End Set
    End Property

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