groupware: fix NPE when flattening durations in slist()

pkg/jmap/api.go

@@ -125,7 +125,11 @@ func RefineResultSlice[A, B any](a []*Result[A], refiner func([]*A, []*SessionSt
 		}
 	})
 	durations := structs.Flatten(structs.Map(a, func(e *Result[A]) []time.Duration {
-		return e.Durations
+		if e != nil {
+			return e.Durations
+		} else {
+			return nil
+		}
 	}))
 	b, bss, bs, bl, err := refiner(payloads, sessionStates, states, languages)
 	return NewResult(b, bss, bs, bl, durations), err

pkg/structs/structs.go

@@ -385,11 +385,15 @@ func Reduce[T any](initialValue T, s []T, reducer func(a, b T) T) T {
 func Flatten[T any](s [][]T) []T {
 	l := 0
 	for _, r := range s {
-		l += len(r)
+		if r != nil {
+			l += len(r)
+		}
 	}
 	result := make([]T, 0, l)
 	for _, r := range s {
-		result = append(result, r...)
+		if r != nil {
+			result = append(result, r...)
+		}
 	}
 	return result
 }