无法读取rxjs中未定义的属性“ map”

Question

我是Angular 6.X和rxjs的新手。我正在尝试实现可观察的点击操作。它通过RestApi提取数据，但是在data ['list']。map的异常之下，我尝试了不同的方法，但没有一个解决。

get(): Observable<ProgramOwnerVulnerability[]> {
        //explicitly do not type this get, due to list structure returned
        return this.http
            .get(this.programListUrl)
            .pipe(map((data) => {
                //map data appropriately into the correct type
                return data['list'].map((programOwnerVulnerabilityJson) => {
                    return new ProgramOwnerVulnerability(programOwnerVulnerabilityJson);
                });
            }));
    }

例外：

core.js:1598 ERROR TypeError: Cannot read property 'map' of undefined
    at MapSubscriber.project (program-owner.service.ts:29)
    at MapSubscriber.push../node_modules/rxjs/_esm5/internal/operators/map.js.MapSubscriber._next (map.js:75)
    at MapSubscriber.push../node_modules/rxjs/_esm5/internal/Subscriber.js.Subscriber.next (Subscriber.js:93)
    at MapSubscriber.push../node_modules/rxjs/_esm5/internal/operators/map.js.MapSubscriber._next (map.js:81)
    at MapSubscriber.push../node_modules/rxjs/_esm5/internal/Subscriber.js.Subscriber.next (Subscriber.js:93)
    at FilterSubscriber.push../node_modules/rxjs/_esm5/internal/operators/filter.js.FilterSubscriber._next (filter.js:85)
    at FilterSubscriber.push../node_modules/rxjs/_esm5/internal/Subscriber.js.Subscriber.next (Subscriber.js:93)
    at MergeMapSubscriber.push../node_modules/rxjs/_esm5/internal/operators/mergeMap.js.MergeMapSubscriber.notifyNext (mergeMap.js:136)
    at InnerSubscriber.push../node_modules/rxjs/_esm5/internal/InnerSubscriber.js.InnerSubscriber._next (InnerSubscriber.js:20)
    at InnerSubscriber.push../node_modules/rxjs/_esm5/internal/Subscriber.js.Subscriber.next (Subscriber.js:93)