Fix exception due to concurrent collection update (#2698) #patch

Sentry event ID: 9ecd1e270bb445c0afe42a4b8872a054

Author: IhateTrains

ImperatorToCK3/CK3/Characters/Character.cs

@@ -829,5 +829,5 @@ CK3LocDB ck3LocDB
 	private readonly CharacterCollection characters;
 	private readonly ConcurrentHashSet<Character> spousesCache = [];
 	private readonly ConcurrentHashSet<Character> concubinesCache = [];
-	private readonly HashSet<Character> childrenCache = new();
+	private readonly ConcurrentHashSet<Character> childrenCache = [];
 }