Bug 138562 Splite extendsexcpetion to seperate sub class
The suggestion is Parent invalid Cycle extends Different type extends Not top level extends
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/elements/OdaDataSet.java
*            the report design
	 * @return an API handle for this element
	 */
	public OdaDataSetHandle handle( Module module )
	{
		if ( handle == null )
		{
			handle = new OdaDataSetHandle( module, this );
		}
		return (OdaDataSetHandle) handle;
	}
	/**
	 * Gets the definition of the extension element.
	 * 
	 * @return the definition of the extension element if found, or null if the
	 *         extended item is not extensible or the extension element is not
	 *         registered in BIRT
	 */
	public ExtensionElementDefn getExtDefn( )
	{
		if ( provider != null )
			return provider.getExtDefn( );
		return null;
	}
	/*
	 * (non-Javadoc)
	 * 
	 * @see org.eclipse.birt.report.model.core.DesignElement#getPropertyDefns()
	 */
	public List getPropertyDefns( )