我有以下代碼，其中包含 2 個(gè)類 MyRange 和 MyCustomValue -class MyRange {    private Long id;    private Double minValue;    private Double maxValue;    // getters and setters    // equals, hashCode and toString    @Override    public boolean equals(Object obj) {        if (this == obj)            return true;        if (obj == null || getClass() != obj.getClass())            return false;        MyRange other = (MyRange) obj;        return Objects.equals(this.id, other.id) &&                Objects.equals(this.minValue, other.minValue) &&               Objects.equals(this.maxValue, other.maxValue);    }}class MyCustomValue {    private String value;    private MyRange myrange;    //getters and setters    // equals, hashCode and toString}如果它value是空的，MyCustomValue我希望它在最后。所以我寫(xiě)了如下的比較器public static final Comparator<MyCustomValue> externalMVComparator = (emv1, emv2) -> {    if(emv1.getValue() != null && emv2.getValue() == null) {        return -1;    } else if (emv1.getValue() == null && emv2.getValue() != null) {        return 1;    } else {        return myrangeMinValueComparator.compare(emv1, emv2);    }}private static final Comparator<MyRange> minValueComparator =  Comparator.nullsLast(Comparator.comparingDouble(value -> value.getMinValue()));private static final Comparator<MyCustomValue> myrangeMinValueComparator = Comparator.nullsLast(Comparator.comparing(MyCustomValue::getMyrange, minValueComparator));上述比較器工作正常。所以我決定改變externalMVComparator如下（即，使用thenComparing更多的可讀性）private static final Comparator<MyCustomValue> valueComparator = Comparator.nullsLast(Comparator.comparing(MyCustomValue::getValue));public static final Comparator<MyCustomValue> externalMVComparator2 = Comparator.nullsLast(valueComparator.thenComparing(myrangeMinValueComparator));但是對(duì)列表進(jìn)行排序，externalMVComparator2結(jié)果為NullPointerException. 我的代碼中有什么錯(cuò)誤？