Merge pull request #2868 from FirelyTeam/fix/contained-resource-witho…

…ut-id-in-cache

contained resource ref cache fix

src/Hl7.Fhir.Base/ElementModel/ScopedNode.cs

@@ -182,7 +182,7 @@ private ReferencedResourceCache getOrInitContainedCache()
             if (AtResource)
             {
                 var referenceEntryPairs = from contained in this.Children("contained")
-                    let id = $"#{contained.Children("id").FirstOrDefault()?.Value as string}"
+                    let id = contained.Children("id").FirstOrDefault()?.Value is string s ? $"#{s}" : null
                     let resource = contained as ScopedNode
                     select new KeyValuePair<string, ScopedNode?>(id, resource);
                 _cache.ContainedResources = new ReferencedResourceCache(referenceEntryPairs);