Fix NPE with loaded save games from past versions.

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();
 }