title: Comparator.nullsFirst with null-safe-comparator
body: <p>I want to have a null-safe comparator, but it does not work:</p>
  
  <pre><code>Comparator&lt;Item&gt; sort_high = (i1, i2)-&gt; Double.compare(i2.getUser().getValue(), i1.getUser().getValue());
  
  items.sort(Comparator.nullsFirst(sort_high));
  </code></pre>
  
  <p>However, I get a NPE, if item.getUser().getValue() (or item.getUser()) is null.</p>
  
  <pre><code>at Item.lambda$1(Item.java:270)
      at java.util.Comparators$NullComparator.compare(Comparators.java:83)
      at java.util.TimSort.countRunAndMakeAscending(TimSort.java:355)
      at java.util.TimSort.sort(TimSort.java:220)
      at java.util.Arrays.sort(Arrays.java:1438)
      at java.util.List.sort(List.java:478)
  </code></pre>
  
  <p>What is wrong?</p>