Fix a locking error by ensuring we lock/unlock on the same object. (#10469)

Author: asvitkine

game-app/game-core/src/main/java/games/strategy/triplea/ui/TerritoryDetailPanel.java

@@ -175,7 +175,8 @@ private void territoryChanged(final @Nullable Territory territory) {
     final String unitsLabel;
 
     // Get the unit information under lock as otherwise they may change on the game thread causing a
-    // ConcurrentModificationException.
+    // ConcurrentModificationException. Use local variable to ensure we lock/unlock the same object.
+    final GameData gameData = this.gameData;
     gameData.acquireReadLock();
     try {
       unitsList = UnitSeparator.getSortedUnitCategories(territory, uiContext.getMapData());