When loading saves from past version, fix up null player references. (#…

…10518)

* When loading saves from past version, fix up null player references.

* Do it in a different place.

* Check for null data and use a method so it can be marked for removal.

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

@@ -363,6 +363,24 @@ public void postDeSerialize() {
     territoryListeners = new CopyOnWriteArrayList<>();
     dataChangeListeners = new CopyOnWriteArrayList<>();
     delegates = new HashMap<>();
+    fixUpNullPlayers();
+  }
+
+  @RemoveOnNextMajorRelease
+  private void fixUpNullPlayers() {
+    // Old save games may have territories and units owned by a different null player object that
+    // has a null data. Update them to the new null player, so that code can rely on getData()
+    // not being null.
+    for (Territory t : getMap().getTerritories()) {
+      if (t.getOwner().getData() == null) {
+        t.setOwner(playerList.getNullPlayer());
+      }
+    }
+    for (Unit u : getUnits()) {
+      if (u.getOwner().getData() == null) {
+        u.setOwner(playerList.getNullPlayer());
+      }
+    }
   }
 
   public interface Unlocker extends Closeable {

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

@@ -150,7 +150,7 @@ public boolean isEmpty() {
 
   /** Returns a Set of all players who have units in this collection. */
   public Set<GamePlayer> getPlayersWithUnits() {
-    // note nulls are handled by PlayerId.NULL_PLAYERID
+    // note nulls are handled by PlayerList.getNullPlayer()
     return units.stream().map(Unit::getOwner).collect(Collectors.toSet());
   }