Bug 202254 Exception is thrown when category values are null and type of X axis is set to DateTime
chart/org.eclipse.birt.chart.engine/src/org/eclipse/birt/chart/computation/DataSetIterator.java
return dda[iCursor++];
	}
	/**
	 * @return
	 */
	public final Calendar nextDateTime( )
	{
		if ( it != null )
		{
			iCursor++;
			return (Calendar) it.next( );
		}
		return ca[iCursor++];
	}
	/**
	 * @return
	 */
	public final String nextText( )
	{
		if ( it != null )
		{
			iCursor++;
			return (String) it.next( );
		}
		return sa[iCursor++];
	}
	/**
	 * @return
	 */
	public final Object nextObject( )