fix: Fix crash when deleting a folder of assets (#2673)

* Fix crash when deleting a folder of assets

Author: NicusorN5

sources/assets/Stride.Core.Assets.Quantum/AssetCompositeHierarchyPropertyGraph.cs

@@ -621,7 +621,7 @@ private void UpdateAssetPartBases()
         var currentParts = Asset.Hierarchy.RootParts.DepthFirst(x => Asset.EnumerateChildParts(x, false)).Select(x => Asset.Hierarchy.Parts[x.Id]);
         foreach (var part in currentParts)
         {
-            if (part.Base is null)
+            if (part.Base?.BasePartAsset is null)
                 continue;
 
             if (Container.TryGetGraph(part.Base.BasePartAsset.Id) is AssetCompositeHierarchyPropertyGraph<TAssetPartDesign, TAssetPart> baseAssetGraph)

sources/assets/Stride.Core.Assets/AssetReferenceDataSerializer.cs

@@ -15,6 +15,9 @@ public override void Serialize(ref AssetReference assetReference, ArchiveMode mo
     {
         if (mode == ArchiveMode.Serialize)
         {
+            if (assetReference is null)
+                return;
+
             stream.Write(assetReference.Id);
             stream.Write(assetReference.Location);
         }