#vulnerable code 
                             @Override
	public ExactRelation rewriteWithSubsampledErrorBounds() {
		ExactRelation newSource = source.rewriteWithSubsampledErrorBounds();
		List<SelectElem> sourceElems = null; // newSource.getSelectList();
		Set<String> colAliases = new HashSet<String>();
		for (SelectElem e : sourceElems) {
			if (e.aliasPresent()) {
				// we're only interested in the columns for which aliases are present.
				// note that every column with aggregate function must have an alias (enforced by ColNameExpr class).
				colAliases.add(e.getAlias());
			}
		}
		
		// we search for error bound columns based on the assumption that the error bound columns have the suffix attached
		// to the original agg columns. The suffix is obtained from the ApproxRelation#errColSuffix() method.
		// ApproxAggregatedRelation#rewriteWithSubsampledErrorBounds() method is responsible for having those columns. 
		List<SelectElem> elemsWithErr = new ArrayList<SelectElem>();
		for (SelectElem e : elems) {
			elemsWithErr.add(e);
			String errColName = errColName(e.getExpr().getText());
			if (colAliases.contains(errColName)) {
				elemsWithErr.add(new SelectElem(new ColNameExpr(errColName), errColName));
			}
		}
		
		ExactRelation r = new ProjectedRelation(vc, newSource, elemsWithErr);
		r.setAliasName(getAliasName());
		return r;
	} 
                            #location 6 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                             @Override
	public ExactRelation rewriteWithSubsampledErrorBounds() {
		ExactRelation r = rewriteWithPartition(true);
		
		// construct a new list of select elements. the last element is __vpart, which should be omitted.
		// newElems and newAggs hold almost the same info; just replicate them to follow the structure
		// of AggregatedRelation-ProjectedRelation.
		List<SelectElem> newElems = new ArrayList<SelectElem>();
		List<Expr> newAggs = new ArrayList<Expr>();
		List<SelectElem> elems = ((ProjectedRelation) r).getSelectElems();
		for (int i = 0; i < elems.size() - 1; i++) {
			SelectElem elem = elems.get(i);
			if (!elem.isagg()) {
				newElems.add(new SelectElem(ColNameExpr.from(elem.getAlias()), elem.getAlias()));
			} else {
				if (elem.getAlias().equals(partitionSizeAlias)) {
					continue;
				}
				
				ColNameExpr est = new ColNameExpr(elem.getAlias(), r.getAliasName());
				ColNameExpr psize = new ColNameExpr(partitionSizeAlias, r.getAliasName());
				
				// average estimate
				Expr averaged = null;
				if (elem.getExpr().isCountDistinct()) {
					// for count-distinct (i.e., universe samples), weighted average should not be used.
					averaged = FuncExpr.round(FuncExpr.avg(est));
				} else {
					// weighted average
					averaged = BinaryOpExpr.from(FuncExpr.sum(BinaryOpExpr.from(est, psize, "*")),
		                    					  FuncExpr.sum(psize), "/");
					if (elem.getExpr().isCount()) {
						averaged = FuncExpr.round(averaged);
					}
				}
				newElems.add(new SelectElem(averaged, elem.getAlias()));
				newAggs.add(averaged);
				
				// error estimation
				// scale by sqrt(subsample size) / sqrt(sample size)
				Expr error = BinaryOpExpr.from(
								BinaryOpExpr.from(FuncExpr.stddev(est), FuncExpr.sqrt(FuncExpr.avg(psize)), "*"),
								FuncExpr.sqrt(FuncExpr.sum(psize)),
								"/");
				error = BinaryOpExpr.from(error, ConstantExpr.from(confidenceIntervalMultiplier()), "*");
				newElems.add(new SelectElem(error, Relation.errorBoundColumn(elem.getAlias())));
				newAggs.add(error);
			}
		}
		
		// this extra aggregation stage should be grouped by non-agg elements except for __vpart
		List<Expr> newGroupby = new ArrayList<Expr>();
		for (SelectElem elem : elems) {
			if (!elem.isagg() && !elem.getAlias().equals(partitionColumnName())) {
				newGroupby.add(ColNameExpr.from(elem.getAlias()));
			}
		}
		r = new GroupedRelation(vc, r, newGroupby);
		r = new AggregatedRelation(vc, r, newAggs);
		r = new ProjectedRelation(vc, r, newElems);
		
		return r;
	}