Wpf Richtextbox clear error

Question

Dear Friends, When I run this code, I get the following error .. system.Executionengineexception. Can anyone guide me as to what I might be doing wrong? Thanks

    Dim lMainParagraph As Paragraph
    Dim lDelimiter As String = String.Empty

    Try


        For x As Integer = 0 To 1



            lMainParagraph = New Paragraph

            For u As Integer = 0 To 1


                lDelimiter = ","

                lMainParagraph.Inlines.Add(New Run("RecordID" & lDelimiter))

                mFlowDocument.Blocks.Add(lMainParagraph)
                ucRichTextbox.Document = mFlowDocument




            Next

        Next


        ucRichTextbox.Document.Blocks.Clear()

    Catch ex As Exception

    End Try