Dynamically Show/Hide Custom Ribbon Tab

Currently I am developing a custom ribbon tab (using the Office CustomUI Editor) that is to show/hide when the user navigates to any page with "Roster" in its name. Below is my XML:

<customUI xmlns="http://schemas.microsoft.com/office/2009/07/customui" onLoad="RibbonOnLoad">
<ribbon>
    <tabs>
      <tab id="customRosterTab" label="Roster Tools" getEnabled="IsRosterTabEnabled">
        <group id="rosterGroup" label="Roster Actions" />
      </tab>
    </tabs>
  </ribbon>
</customUI>

I am using the VB script in the ThisWorkbook module:

Private Sub Workbook_SheetActivate(ByVal Sh As Object)
    If Not ribbonUI Is Nothing Then
        ribbonUI.Invalidate ("customRosterTab") ' Forces the ribbon to refresh, calling IsRosterTabEnabled
    End If
End Sub

and the script below in a separate module:

Dim ribbonUI As IRibbonUI

' Initialize ribbon UI
Sub RibbonOnLoad(ribbon As IRibbonUI)
    Set ribbonUI = ribbon
    MsgBox "Ribbon has loaded successfully!"
End Sub

' Callback function to enable/disable the tab based on sheet name
Function IsRosterTabEnabled(control As IRibbonControl) As Boolean
    ' Enable tab if the active sheet's name contains "Roster"
    IsRosterTabEnabled = InStr(1, ActiveSheet.Name, "Roster", vbTextCompare) > 0
End Function

Unfortunately, every time I change to any sheet containing "Roster" in its name, I get an "Object Required" error on this line "If Not ribbonUI Is Nothing Then" (because ribbonUI is empty).

I'm not sure how to fix this. Can anybody provide any insight?

Solution

Turns out, using the Tag attribute is ideal. Here's what I did:

In a standard module I put this

Option Explicit

Dim Rib As IRibbonUI
Dim MyTag As String

'Callback for customUI.onLoad
Sub RibbonOnLoad(ribbon As IRibbonUI)
    Set Rib = ribbon
End Sub

Sub GetVisible(control As IRibbonControl, ByRef visible)
    If control.Tag Like MyTag Then
        visible = True
    Else
        visible = False
    End If
End Sub

Sub RefreshRibbon(Tag As String, Optional TabID As String)
    MyTag = Tag
    If Rib Is Nothing Then
        MsgBox "Error, restart your workbook"
    Else
        Rib.Invalidate
        If TabID <> "" Then Rib.ActivateTab TabID
    End If
End Sub

Then, in the ThisWorkbook module

Private Sub Workbook_SheetActivate(ByVal Sh As Object)
        
        Select Case True
            Case Sh.Name Like "*Roster"
                Call RefreshRibbon(Tag:="RosterTab", TabID:="tabRosterSheet")        
            Case Else
                Call RefreshRibbon(Tag:="")
        End Select

End Sub

And in my XML

<tab id="tabRosterSheet" label="Roster Sheet Tools" insertBeforeMso="TabInsert" tag="RosterTab" getVisible="GetVisible">

Works like a charm!