Check for empty "last_status" before sorting DM column (#9207)

* Check for empty "last_status" before sorting

* Small touchups for codeclimate

app/javascript/mastodon/reducers/conversations.js

@@ -56,7 +56,13 @@ const expandNormalizedConversations = (state, conversations, next) => {
 
         list = list.concat(items);
 
-        return list.sortBy(x => x.get('last_status'), (a, b) => compareId(a, b) * -1);
+        return list.sortBy(x => x.get('last_status'), (a, b) => {
+          if(a === null || b === null) {
+            return -1;
+          }
+
+          return compareId(a, b) * -1;
+        });
       });
     }