Vb multithreading with “specified cast is not valid” error

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VB

Hi all..
If I run the code below without threading everything works like it should, but when I start it as a new thread then I receive ‘System.InvalidCastException’ error with “Specified cast is not valid”

Thanks in advance for any help you can provide.

Dim thread1 As New Thread(AddressOf th10)

Private Sub Form3_Load(sender As Object, e As EventArgs) Handles MyBase.Load
    CheckForIllegalCrossThreadCalls = False
    Button1.PerformClick()
End Sub

Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
    thread1.Start()
End Sub

Public Sub th10()
    WebBrowser3.Navigate("https://www.somewebsite.com")
    While WebBrowser3.ReadyState <> WebBrowserReadyState.Complete
        Application.DoEvents()
    End While

    Dim pos4 As Long
    Dim bp2 As String

    Dim value1 As String = WebBrowser3.Document.Body.InnerText

    pos4 = InStr(pos4 + 1, value1, "sometextfromwebsite", vbTextCompare)
    bp2 = value1.Substring(pos4)
    Dim startIndex1 As Integer = 17
    Dim length1 As Integer = 16
    Dim substring1 As String = bp2.Substring(startIndex1, length1)

    TextBox3.Text = substring1
End Sub

What I have tried:

This error shows at “While WebBrowser3.ReadyState…” and when I change it to;

Static start As Single
start = VB.Timer()
Do While VB.Timer() < start + 5
Application.DoEvents()
Loop

(I don’t like the idea to specify a time for page load, in some cases it might take longer)

…it seems to be fine, but then I receive the same error on “Dim value1 As String = WebBrowser3.Document.Body.InnerText”