Error using Page.PreviousPage

[David C]

I have a dual purpose web page that is giving the error below when coming
from one web page but not another. I don't really know where to look. Both
pages are calling this page via NavigateURL setting on a LinkButton. Can
someone help direct me to a solution? Thanks.

Error executing child request for handler 'ASP.frmpropertyclosings_aspx'.

Part of the code where the error is occurring is below.

If Not Page.IsPostBack Then
'First page visit
If Not Page.PreviousPage Is Nothing Then
'Coming from frmProperty page
Dim strFolderPath As String = ""
Dim strFolderName As String = ""
Select Case strListType
Case "list"
SourceTextBox =
CType(PreviousPage.FindControl("txtAddressTop"), TextBox)
strFolderName =
UtilClass.ReplaceFolderChars(SourceTextBox.Text)
strFolderPath = VirtualListingRoot & strFolderName

Case "close"
SourceTextBox =
CType(PreviousPage.FindControl("txtClosingName"), TextBox)
strFolderName =
UtilClass.ReplaceFolderChars(SourceTextBox.Text)
strFolderPath = VirtualClosedRoot & strFolderName

Case Else 'expired
SourceTextBox =
CType(PreviousPage.FindControl("txtAddressTop"), TextBox)
strFolderName =
UtilClass.ReplaceFolderChars(SourceTextBox.Text)
strFolderPath = VirtualExpiredRoot & strFolderName
End Select
txtAddress.Text = strFolderName

The Stack Trace is below.

Stack Trace:
at System.Web.HttpServerUtility.ExecuteInternal(IHttpHandler handler,
TextWriter writer, Boolean preserveForm, Boolean setPreviousPage,
VirtualPath path, VirtualPath filePath, String physPath, Exception error,
String queryStringOverride)
at System.Web.HttpServerUtility.Execute(IHttpHandler handler, TextWriter
writer, Boolean preserveForm, Boolean setPreviousPage)
at System.Web.UI.Page.get_PreviousPage()
at frmListingDocs.Page_Load(Object sender, EventArgs e) in
http://server/rfpweboffice/frmListingDocs.aspx.vb:line 54
at System.Web.UI.Control.OnLoad(EventArgs e)
at System.Web.UI.Control.LoadRecursive()
at System.Web.UI.Page.ProcessRequestMain(Boolean
includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
at System.Web.UI.Page.ProcessRequest(Boolean
includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
at System.Web.UI.Page.ProcessRequest()
at System.Web.UI.Page.ProcessRequestWithNoAssert(HttpContext context)
at System.Web.UI.Page.ProcessRequest(HttpContext context)
at ASP.frmlistingdocs_aspx.ProcessRequest(HttpContext context)
at
System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean&
completedSynchronously)

-David