Imports System.Collections.Concurrent

Module declarations
    Public cb1Pages As New ConcurrentBag(Of HtmlDocument)
    Public cbUrls As New ConcurrentBag(Of String)
...
...
...
End Module

Imports System.Threading

Public Class Form1
    Private WithEvents wb As New WebBrowser
    Private uc As New _1stCollector
    Private thUrl As Thread
    ...
    ...

    Private Sub Form1_Shown(sender As Object, e As EventArgs) Handles Me.Shown
        ...
        thUrl = New Thread(AddressOf uc.GStart)
        thUrl.IsBackground = True
        thUrl.Start()
        wb.Navigate("https://www.google.ru/search?q=" & TextBox1.Text.Replace(" ", "+"), False)
    End Sub

    Private Sub wb_DocumentCompleted(sender As Object, e As WebBrowserDocumentCompletedEventArgs) Handles wb.DocumentCompleted
        cb1Pages.Add(wb.Document)

        Dim nxt As HtmlElement = wb.Document.GetElementById("pnnext")
        If Not nxt Is Nothing Then
            nxt.InvokeMember("click")
        End If
    End Sub
End Class

Imports System.Collections.Concurrent
Imports System.Threading

Public Class _1stCollector
    Public Sub GStart()
        While True
            Try
                If Not cb1Pages.IsEmpty Then
                    Dim coll_doc As HtmlDocument
                    If cb1Pages.TryTake(coll_doc) Then
                        For Each elem As HtmlElement In coll_doc.GetElementById("rso").GetElementsByTagName("h3")  '' ругается здесь
                            For Each alem As HtmlElement In elem.GetElementsByTagName("a")
                                cbUrls.Add(alem.GetAttribute("href"))
                                Console.WriteLine(alem.GetAttribute("href"))
                            Next
                        Next
                    End If
                End If
                Thread.Sleep(200)
            Catch ex As Exception
                Console.WriteLine("1st " & ex.Message)
            End Try
        End While
    End Sub
End Class

Dim coll_doc As IHTMLDocument2 = DirectCast(New HTMLDocument, IHTMLDocument2)