When creating a CreatureParty with a size, don't just reserve the space, but append nullptr as well.

src/Battling/Models/CreatureParty.hpp

@@ -9,7 +9,11 @@ namespace CreatureLib::Battling {
         ArbUt::UniquePtrList<Creature> _party;
 
     public:
-        CreatureParty(size_t size) noexcept : _party(size) {}
+        CreatureParty(size_t size) noexcept : _party(size) {
+            for (size_t i = 0; i < size; i++) {
+                _party.Append(nullptr);
+            }
+        }
         CreatureParty(ArbUt::List<Creature*> party) noexcept : _party(party.GetStdList()) {}
         CreatureParty(std::initializer_list<Creature*> party) noexcept : _party(party) {}