Fix for 2 NREs related to assets

[probablykory]

Fix for 2 NREs that seem to occur while running Jotunn mods alongside Krumpac's Reforged modpack.

[probablykory]

Tested normal operation (0.217.46 with MoreCrossbows and Jotunn), and this change seems to be benign. Tested with Reforged, and with the change it works fine. Without the change (eg tested with Jotunn 2.19.3 to verify) it does indeed throw an NRE.

[Error  :Jotunn.Managers.GUIManager] System.NullReferenceException: Object reference not set to an instance of an object
  at Jotunn.Managers.PrefabManager+Cache.GetPrefab (System.Type type, System.String name) [0x0002a] in D:\Projects\Jotunn\JotunnLib\Managers\PrefabManager.cs:444
  at Jotunn.Managers.PrefabManager+Cache.GetPrefab[T] (System.String name) [0x00000] in D:\Projects\Jotunn\JotunnLib\Managers\PrefabManager.cs:472
  at Jotunn.Managers.GUIManager.InitializeAssets () [0x002ec] in D:\Projects\Jotunn\JotunnLib\Managers\GUIManager.cs:423

Full log attached.

[MSchmoecker]

thanks, looks good now