My object packages are randomly breaking

Hi I am experiencing a rather frustrating bug and not sure what's causing it. I appreciate any help. 

So far this has happened twice in the past two days, with two seperate packages. The thing these packages have in common is that they started as clones of the base game typewriter.

I've made plenty of cc objects, so this is not a basic issue. I imported the mesh and swatches normally, and everything looked fine. I save and close the window.  But then the next time I try to open the package, the mesh, rig, swatches etc are gone, and I get the following error:


Sims 4 Studio 1.4.1.1 (Aurora)
System.NullReferenceException: Object reference not set to an instance of an object
at S4Studio.ViewModels.ObjectCustomContentViewModel.ModelKeys (S4Studio.ViewModels.Generic.Sims4SwatchContent sr) [0x0000b] in <618d2d56b6b84107a65659745d61d886>:0
at S4Studio.ViewModels.Generic.BuyBuildSims4CustomContent.<get_Models>b__47_0 (S4Studio.ViewModels.Generic.Sims4SwatchContent x) [0x00000] in <618d2d56b6b84107a65659745d61d886>:0
at System.Linq.Enumerable+SelectManySingleSelectorIterator`2[TSource,TResult].MoveNext () [0x00051] in <984f4182e8a347e6962e41942e07db08>:0
at System.Linq.Enumerable+DistinctIterator`1[TSource].MoveNext () [0x0009a] in <984f4182e8a347e6962e41942e07db08>:0
at S4Studio.ViewModels.Generic.BuyBuildSims4CustomContent.get_Models () [0x00066] in <618d2d56b6b84107a65659745d61d886>:0
at S4Studio.ViewModels.Generic.BuyBuildSims4CustomContent.get_Lods () [0x00016] in <618d2d56b6b84107a65659745d61d886>:0
at S4Studio.ViewModels.Generic.BuyBuildSims4CustomContent.get_SelectedLodLevel () [0x00008] in <618d2d56b6b84107a65659745d61d886>:0
at S4Studio.ViewModels.ObjectSwatchItem.get_SwatchTextures () [0x00006] in <618d2d56b6b84107a65659745d61d886>:0
at S4Studio.ObjectTextureEditorViewController.set_Swatch (S4Studio.ViewModels.Generic.Sims4SwatchContent value) [0x00057] in <618d2d56b6b84107a65659745d61d886>:0
at S4Studio.StudioViewController`1[TCustomContentViewModel].SetSwatch () [0x00028] in <618d2d56b6b84107a65659745d61d886>:0
at S4Studio.ObjectStudioViewController.SetSwatch () [0x00000] in <618d2d56b6b84107a65659745d61d886>:0
at S4Studio.StudioViewController`1[TCustomContentViewModel].ViewWillAppear () [0x00039] in <618d2d56b6b84107a65659745d61d886>:0


I have previously saved backups of the same package that still function, so I suspect this is a saving issue, but I'm not sure what's triggering it, and so far it has only happened with the typewrite clones. However, the typewriter is the best base I could find since I need an object with 2 diffuse maps.