ngInit.ts

 ngOnInit() {
    console.log('ngOnInit() invokes with default group', this.defaultGroup);
    const filter$: Observable<string> = this.filterControl.valueChanges.pipe(
      startWith(''),
      debounceTime(100), 
      distinctUntilChanged(),
      share()
    );
    this.dayOverViewByGroup$ = this.attendanceService.getDayOverViewByGroupIdObservable(moment().format("YYYY-MM-DD"), 3).pipe( 
      // If the Observable from your service can emit 'null' and you do
      // map(days => days.map(..)) you will get an error 
      // so you might have to handle this case here
  
      // 1. Option: filter out 'null' and 'undefined' values before your 'map' and don't emit anything
      //filter(Boolean)
  
      // 2. Option: handle 'null' and 'undefined' in your map function
      map(days => {     
        if (days && days === undefined) {
          console.log('days inside of', days)
          days.map(day => {
            console.log('day in map', day);
            return day;
          })
        } else {
          // do something else or don't do anything at all, i.e. remove the else case
          return false;
        }
      })
    )                                                                                                                              ```