Fix loading of save games following nullPlayer refactoring. (#10496)

* Fix loading of save games following nullPlayer refactoring.

* Fix imports.

game-app/game-core/src/main/java/games/strategy/engine/data/PlayerList.java

@@ -3,6 +3,8 @@
 import com.google.common.annotations.VisibleForTesting;
 import games.strategy.triplea.Constants;
 import games.strategy.triplea.util.PlayerOrderComparator;
+import java.io.IOException;
+import java.io.ObjectInputStream;
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.HashMap;
@@ -14,6 +16,7 @@
 import java.util.stream.Stream;
 import lombok.Getter;
 import lombok.ToString;
+import org.triplea.java.RemoveOnNextMajorRelease;
 
 /** Wrapper around the set of players in a game to provide utility functions and methods. */
 @ToString
@@ -22,19 +25,31 @@ public class PlayerList extends GameDataComponent implements Iterable<GamePlayer
 
   // maps String playerName -> PlayerId
   private final Map<String, GamePlayer> players = new LinkedHashMap<>();
-  @Getter private final GamePlayer nullPlayer;
+  @Getter private GamePlayer nullPlayer;
 
   public PlayerList(final GameData data) {
     super(data);
-    nullPlayer =
-        new GamePlayer(Constants.PLAYER_NAME_NEUTRAL, true, false, null, false, data) {
-          private static final long serialVersionUID = 1;
-
-          @Override
-          public boolean isNull() {
-            return true;
-          }
-        };
+    nullPlayer = createNullPlayer(data);
+  }
+
+  @RemoveOnNextMajorRelease
+  private void readObject(final ObjectInputStream in) throws IOException, ClassNotFoundException {
+    in.defaultReadObject();
+    // Old save games may not have nullPlayer set.
+    if (nullPlayer == null) {
+      nullPlayer = createNullPlayer(getData());
+    }
+  }
+
+  private GamePlayer createNullPlayer(GameData data) {
+    return new GamePlayer(Constants.PLAYER_NAME_NEUTRAL, true, false, null, false, data) {
+      private static final long serialVersionUID = 1;
+
+      @Override
+      public boolean isNull() {
+        return true;
+      }
+    };
   }
 
   @VisibleForTesting