Implemented the "container" property for the PiperApp app in the app target.

Piper/Sources/App/PiperApp.swift

@@ -14,16 +14,14 @@ struct PiperApp: App {
     
     // MARK: Properties
 
-    var sharedModelContainer: ModelContainer = {
-        let schema = Schema([
-            Item.self,
-        ])
-        let modelConfiguration = ModelConfiguration(schema: schema, isStoredInMemoryOnly: false)
-
+    private var container = {
         do {
-            return try ModelContainer(for: schema, configurations: [modelConfiguration])
+            return try ModelContainer(
+                for: .entities,
+                configurations: [ModelConfiguration()]
+            )
         } catch {
-            fatalError("Could not create ModelContainer: \(error)")
+            fatalError("ERROR: ModelContainer has not been created => \(error)")
         }
     }()