title: DotNetRDF writing to RDF Exception errors
body: <p>How do I debug/validate my SKOS / RDF?
  What tools are available on Windows 10? </p>
  
  <p>I've been building SKOS /RDF using DotNetRDF </p>
  
  <p>The Problem is:
  When I try to output to a file I can save to .ttl but get <code>"System.UriFormatException: 'Invalid URI: The URI is empty.'</code> when saving to .rdf</p>
  
  <p>I have no idea how to find this empty URI opening it in a txt editor isn't helpful</p>
  
  <pre><code>     //  This Works
      CompressingTurtleWriter ttlwriter = new CompressingTurtleWriter();
      ttlwriter.Save(g, @"C:\Temp\DotNetRDFTestG.ttl");
  
      //  this errors
      RdfXmlWriter rdfxmlwriter = new RdfXmlWriter();
      rdfxmlwriter.Save(g, @"C:\Temp\MydotNetRdf.rdf");
  </code></pre>
  
  <p>I don't know if this is related but:</p>
  
  <p>I can add the .ttl file to Apache Jena Fuseki but when I try and view it in SKOSMOS I get the <code>Undefined index: http://www.w3.org/2004/02/skos/core#Concept</code> in the Apache error log.</p>
  
  <p>Are there any tutorials that include DotNETRDF, Apache Jena Fuseki, and SKOSMOS?</p>