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
public final Object next( )
	{
		if ( iCursor >= iRowCount )
		{
			throw new RuntimeException( new ChartException( ChartEnginePlugin.ID,
					ChartException.COMPUTATION,
					"exception.out.of.bounds", 
					Messages.getResourceBundle( ) ) );
		}
		if ( it != null )
		{
			iCursor++;
			return it.next( );
		}
		if ( iDataType == IConstants.NUMERICAL )
		{
			if ( iContentType == IConstants.NON_PRIMITIVE_ARRAY )
			{
				return nextDouble( );
			}
			else if ( iContentType == IConstants.PRIMITIVE_ARRAY )
			{
				return new Double( nextPrimitiveDouble( ) );
			}
		}
		else if ( iDataType == IConstants.DATE_TIME )
		{
			if ( iContentType == IConstants.NON_PRIMITIVE_ARRAY )
			{
				return nextDateTime( );