VB.NET Extract Text From PDF with iText7

Question

I get the following error: "The object reference was not set to an object instance." in the line PdfTextExtractor. I installed the packages: itext, itext.commons und itext7

Imports iText.Kernel.Pdf
Imports iText.Kernel.Pdf.Canvas.Parser
Imports iText.Kernel.Pdf.Canvas.Parser.Listener

Public Function ReadPdf(Quelle As String)
        Dim pdfReader As PdfReader = New PdfReader(Quelle)
        Dim pdfDoc As PdfDocument = New PdfDocument(pdfReader)

        For page As Integer = 1 To pdfDoc.GetNumberOfPages()
            Dim strategy As ITextExtractionStrategy = New SimpleTextExtractionStrategy()
            Dim pageContent As String = PdfTextExtractor.GetTextFromPage(pdfDoc.GetPage(page), strategy)
        Next

        pdfDoc.Close()
        pdfReader.Close()
    End Function

It seems to be a systemic problem. Do i need further packages to install?