Fix NPE with loaded save games from past versions. (#11783)

The issue is that old saved games may save references to the GamePlayer on various objects, including pending battles.

Since 2.6 has an invariant that all GamePlayer objects have a non-null getData(), which was not true in 2.5 and earlier, we need to update these after loading the game. In particular, we need to fix up all references to the null player with the instance from the game data.

This was already being done for units and territories via GameData::fixUpNullPlayers(). This adds it also for pending battles.

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

@@ -395,6 +395,14 @@ private void fixUpNullPlayers() {
     }
   }
 
+  @RemoveOnNextMajorRelease
+  public void fixUpNullPlayersInDelegates() {
+    BattleDelegate battleDelegate = (BattleDelegate) getDelegate("battle");
+    if (battleDelegate != null) {
+      battleDelegate.getBattleTracker().fixUpNullPlayers(playerList.getNullPlayer());
+    }
+  }
+
   public interface Unlocker extends Closeable {
     @Override
     void close();

game-app/game-core/src/main/java/games/strategy/engine/framework/GameDataManager.java

@@ -74,6 +74,7 @@ public static Optional<GameData> loadGameUncompressed(final InputStream is) {
       final GameData data = (GameData) input.readObject();
       data.postDeSerialize();
       loadDelegates(input, data);
+      data.fixUpNullPlayersInDelegates();
       return Optional.of(data);
     } catch (final Throwable e) {
       log.warn(

game-app/game-core/src/main/java/games/strategy/triplea/delegate/battle/AbstractBattle.java

@@ -27,6 +27,7 @@
 import java.util.UUID;
 import java.util.stream.Collectors;
 import lombok.Getter;
+import org.triplea.java.ObjectUtils;
 import org.triplea.java.RemoveOnNextMajorRelease;
 import org.triplea.java.collections.CollectionUtils;
 import org.triplea.java.collections.IntegerMap;
@@ -42,7 +43,7 @@ abstract class AbstractBattle implements IBattle {
   boolean headless = false;
 
   @Getter final Territory battleSite;
-  final GamePlayer attacker;
+  GamePlayer attacker;
   GamePlayer defender;
   final BattleTracker battleTracker;
   int round = 1;
@@ -227,6 +228,16 @@ public GamePlayer getDefender() {
     return defender;
   }
 
+  @Override
+  public void fixUpNullPlayer(GamePlayer nullPlayer) {
+    if (attacker.isNull() && !ObjectUtils.referenceEquals(attacker, nullPlayer)) {
+      attacker = nullPlayer;
+    }
+    if (defender.isNull() && !ObjectUtils.referenceEquals(defender, nullPlayer)) {
+      defender = nullPlayer;
+    }
+  }
+
   public void setHeadless(final boolean headless) {
     this.headless = headless;
   }

game-app/game-core/src/main/java/games/strategy/triplea/delegate/battle/BattleTracker.java

@@ -51,6 +51,7 @@
 import java.util.stream.Collectors;
 import javax.annotation.Nullable;
 import lombok.extern.slf4j.Slf4j;
+import org.triplea.java.RemoveOnNextMajorRelease;
 import org.triplea.java.collections.CollectionUtils;
 import org.triplea.java.collections.IntegerMap;
 import org.triplea.sound.ISound;
@@ -176,6 +177,13 @@ private boolean didThesePlayersJustGoToWarThisTurn(final GamePlayer p1, final Ga
     return false;
   }
 
+  @RemoveOnNextMajorRelease
+  public void fixUpNullPlayers(GamePlayer nullPlayer) {
+    for (var b : pendingBattles) {
+      b.fixUpNullPlayer(nullPlayer);
+    }
+  }
+
   void clearFinishedBattles(final IDelegateBridge bridge) {
     for (final IBattle battle : List.copyOf(pendingBattles)) {
       if (FinishedBattle.class.isAssignableFrom(battle.getClass())) {

game-app/game-core/src/main/java/games/strategy/triplea/delegate/battle/IBattle.java

@@ -17,6 +17,7 @@
 import lombok.AllArgsConstructor;
 import lombok.Getter;
 import lombok.ToString;
+import org.triplea.java.RemoveOnNextMajorRelease;
 
 /** Represents a battle. */
 public interface IBattle extends Serializable {
@@ -165,5 +166,8 @@ void unitsLostInPrecedingBattle(
 
   GamePlayer getDefender();
 
+  @RemoveOnNextMajorRelease
+  void fixUpNullPlayer(GamePlayer nullPlayer);
+
   UUID getBattleId();
 }