title: ArgumentNullException on ReadXML
body: <p>I have a problem with my DataSet serialization to XML file. Every time I try to load the XML, the ReadXML() function throws the ArgumentNullException. What is interesting, this issue occurs only when I load it to the empty dataset. When I fill tables in my DataSet and then try to replace them loading DataSet from XML file, there is no Exception thrown. My DataTables are connected with relations. I do not use XMLScheme, because then my XML file never loads, even if I replace the existing dataset.</p>
  
  <p>When the exception is thrown I have one table partly loaded: first record is loaded correctly and second one is filled by null values and StrongTypingExceptions. </p>
  
  <p>Done anyone know what is going on ?</p>
  
  <p>//edit:</p>
  
  <p>Writing:</p>
  
  <pre><code>config.WriteXml(FileName,XmlWriteMode.IgnoreSchema);
  </code></pre>
  
  <p>Reading:</p>
  
  <pre><code>config.ReadXml(FileName,XmlWriteMode.IgnoreSchema);
  </code></pre>
  
  <p>I've tried all modes.</p>
  
  <p>When schema is reading:</p>
  
  <blockquote>
    <p>[System.Data.DataException]   {"invalid 'key' node inside constraint:
    FK_SapTable_SapFieldFilter."} System.Data.DataException</p>
  </blockquote>
  
  <p>I have two tables connected by relation FK_SapTable_SapFieldFilter. In table "SapTable" i have field TableId as key - it is never null. It is connected to TableId field in SapFieldFilter. My SapFieldFilter is an empty table for now, but it shouldn't make a problem:</p>
  
  <p><a href="http://img5.imageshack.us/img5/9469/beznazwy4v.png" rel="nofollow">My relation -> pic</a></p>