Access label control from masterpage throws nullreferenceexception - vb.net

Question

enter code hereI am trying to assign a value to label in a Master Page using vb.net (not my first language-lol). I have followed info from these two references on how to access content from masterpage:
1.[https://msdn.microsoft.com/en-us/library/c8y19k6h(v=vs.90).aspx][1]
2.[nullreferenceexception was unhandled by user code in Master Page
I am getting Null Reference Exception on BUT ONLY when I try to call the method I created for it from another class. It works fine when I call it from the same class the method is in. I wrote a method from the starting page/class, WebForm1:
Public Sub PageIdentity(ByVal pageId As String)
' Gets a reference to a Label control inside a ContentPlaceHolder
Dim mpContentPlaceHolder As ContentPlaceHolder
'Dim mpTextBox As TextBox
Dim mpLabel As Label
mpContentPlaceHolder = CType(Master.FindControl("MainContent"), ContentPlaceHolder)*'Null Reference Exception here*
If Not mpContentPlaceHolder Is Nothing Then
mpLabel = CType(mpContentPlaceHolder.FindControl("lblPageIdentifier"), Label)
If Not mpLabel Is Nothing Then
mpLabel.Text = pageId
End If
End If
End Sub
when I call this function from another class in page load like this it throws null ref ex:
Dim oWebForm1 As WebForm1 = New WebForm1()
oWebForm1.PageIdentity("SomeText")
I must be doing something wrong in creating the instance? I tryed writing it as a Public Shared Function first but that created other problems. Can anyone help?
UPDATE: #Joey I replaced the commented line of code below as you suggested and put the sub in the master page code behind, Site1.Master.vb:
'mpContentPlaceHolder = CType(Master.FindControl("MainContent"), ContentPlaceHolder)
mpContentPlaceHolder = CType(Me.FindControl("MainContent"), ContentPlaceHolder)
I also ensured that the page directive on other pages included:
<%# MasterType VirtualPath = "~/Site1.Master" %>
On a test page, I called the sub like so:
Dim _SiteMaster As MasterPage = TryCast(Me.Master, MasterPage)
_SiteMaster.PageIdentity("SomeText")
I am getting blue squiggly line error, message: "PageIdentity is not a member of System.Web.UI.MasterPage"
The Master page inherits like this:
Public Partial Class Site1
Inherits System.Web.UI.MasterPage
The page calling the sub inherits like this
Public Class WebForm1
Inherits System.Web.UI.Page

Answer 1

Since you require the PageIdentity subroutine to be somewhat global you can move it into your master page. The master page can then be called by any page that references it. Use the following code on any page to call the PageIdentity routine:
Dim _SiteMaster As Site1 = TryCast(Me.Master, Site1)
_SiteMaster.PageIdentity("SomeText")