Packaged game crash when saving classes

Hi guys,

So in my game there is a system for saving loadouts to files using the save game system in UE4.
The LoadoutSaveGame objects contain an array of loadout structs

When saving these loadout structs the packaged game will crash with this error:

Fatal error: [File:D:\Build++UE4+Release-4.18+Compile\Sync\Engine\Source\Runtime\Core\Private\Serialization\Archive.cpp] [Line: 282] 
FArchive does not support FSoftObjectPtr serialization. Use FArchiveUObject instead.

This crash does not occur in the editor version of the game.
I first tried removing all SoftObjectPtrs in all of the classes within the loadout struct. The only 2 were in the Weapon Base class. Despite their removal the crash still occurs in the packaged game however. Is this a problem with my code or an issue with UE4 that needs a workaround or fix?