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
reset( );
		Object o = null;
		while ( hasNext( ) )
		{
			o = next( );
		}
		return o;
	}
	/**
	 * Frees all references to data held internally in this structure
	 */
	public final void clear( )
	{
		dda = null;
		ca = null;
		da = null;
		la = null;
		oa = null;
		sa = null;
		if ( co != null )
		{
			
			co = null;
		}
		iContentType = IConstants.UNDEFINED;
		iDataType = IConstants.UNDEFINED;
		iRowCount = 0;
		iCursor = 0;
		cReused = null;