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/model/data/util/DataSwitch.java
case DataPackage.SAMPLE_DATA :
			{
				SampleData sampleData = (SampleData) theEObject;
				Object result = caseSampleData( sampleData );
				if ( result == null )
					result = defaultCase( theEObject );
				return result;
			}
			case DataPackage.SERIES_DEFINITION :
			{
				SeriesDefinition seriesDefinition = (SeriesDefinition) theEObject;
				Object result = caseSeriesDefinition( seriesDefinition );
				if ( result == null )
					result = defaultCase( theEObject );
				return result;
			}
			case DataPackage.SERIES_GROUPING :
			{
				SeriesGrouping seriesGrouping = (SeriesGrouping) theEObject;
				Object result = caseSeriesGrouping( seriesGrouping );
				if ( result == null )
					result = defaultCase( theEObject );
				return result;
			}
			case DataPackage.STOCK_DATA_SET :
			{
				StockDataSet stockDataSet = (StockDataSet) theEObject;
				Object result = caseStockDataSet( stockDataSet );
				if ( result == null )
					result = caseDataSet( stockDataSet );