Description:
------------
getSequence() returns null with a complexType defined without mixed="true".

Reproduce code:
---------------
<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
  xmlns:letter="http://letterSchema"
  targetNamespace="http://letterSchema">
  <xs:element name="letters" type="FormLetter"/>
  <xs:complexType name="FormLetter">
      <xs:sequence>
      <xs:element name="date" minOccurs="1" type="xs:string"/>
      <xs:element name="firstName" minOccurs="1" type="xs:string"/>
      <xs:element name="lastName" minOccurs="1" type="xs:string"/>
    </xs:sequence>
  </xs:complexType>
</xs:schema>

<?php
/**
 * Create an XML document from scratch
 */
try {
   $xmldas = SDO_DAS_XML::create("letter.xsd");
   try {
       $doc = $xmldas->createDocument();
       $rdo = $doc->getRootDataObject();
       $seq = $rdo->getSequence();
       $seq->insert("April 09, 2005", NULL, 'date');
       $seq->insert("Tarun", NULL, "firstName");
       $seq->insert("Nayaraaa", NULL, "lastName");
       $rdo->lastName = "Nayar";
       print($xmldas->saveString($doc));
   } catch (SDO_Exception $e) {
       print($e);
   }
} catch (SDO_Exception $e) {
   print("Problem creating an XML document: " . $e->getMessage());
}
?>


Expected result:
----------------
<?xml version="1.0" encoding="UTF-8"?>
<letters xmlns="http://letterSchema" xmlns:tns="http://letterSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"><date>April 09, 2005</date><firstName>Tarun</firstName><lastName>Nayar</lastName></letters>


Actual result:
--------------
Fatal error: Call to a member function insert() on a non-object in letter.php on line 11