raise err when collection is missing from cache

lib/services/collections_service.dart

@@ -213,6 +213,9 @@ class CollectionsService {
   Uint8List getCollectionKey(int collectionID) {
     if (!_cachedKeys.containsKey(collectionID)) {
       final collection = _collectionIDToCollections[collectionID];
+      if (collection == null) {
+        throw AssertionError('collectionID $collectionID is not cached');
+      }
       _cachedKeys[collectionID] = _getDecryptedKey(collection);
     }
     return _cachedKeys[collectionID];
@@ -458,8 +461,6 @@ class CollectionsService {
     });
   }
 
-
-
   void _validateMoveRequest(
       int toCollectionID, int fromCollectionID, List<File> files) {
     if (toCollectionID == fromCollectionID) {

lib/ui/create_collection_page.dart

@@ -286,7 +286,8 @@ class _CreateCollectionPageState extends State<CreateCollectionPage> {
     final dialog = createProgressDialog(context, "restoring files...");
     await dialog.show();
     try {
-      await CollectionsService.instance.restore(toCollectionID, widget.selectedFiles.files?.toList());
+      await CollectionsService.instance
+          .restore(toCollectionID, widget.selectedFiles.files?.toList());
       RemoteSyncService.instance.sync(silently: true);
       widget.selectedFiles?.clearAll();
       await dialog.hide();