wpf flowdocumentreader line highlighting
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I'm currently using a FlowDocumentReader to display a text file. I would like to highlight a certain lines in the file when it is displayed. I'm trying to do this with the following code

    FlowDocument flowDoc1 = new FlowDocument();

    Paragraph para1 = new Paragraph();
    para1.FontSize = 12;
    para1.Inlines.Add(new Run(allText));
    flowDoc1.Blocks.Add(para1);

    reader.Document = flowDoc1;

    TextPointer start = reader.Document.ContentStart;

    start = start.GetLineStartPosition(0);
    end = start.GetLineStartPosition(1);    This here returns null

    TextRange range = new TextRange(start, end);

    SolidColorBrush brush = new SolidColorBrush(Color.FromRgb(255, 250, 205));

    range.ApplyPropertyValue(TextElement.BackgroundProperty, brush);

I'm trying to get the first line of the file to be highlighted. The problem is that the call to end = start.GetLineStartPosition(1) is returning null and I'm not sure why. Is this the correct approach to highlight the first line? Does anyone know what I'm doing wrong? The file has about a 1000 lines in it. I also tried replacing "\r" with "" in case the cr lf combination was messing up the end of line markers but this doesn't solve my problem