Fix folder id generation. (#1148)

The `attempt` variable was wrongly increased before leading to this
error (too many recursive calls):
```
packages/filesharing_logic/src/models/folder.dart 125:46                 generateFolderID
5443
packages/filesharing_logic/src/models/folder.dart 126:14                 generateFolderID
packages/sharezone/filesharing/widgets/file_sharing_page_fab.dart 75:34  <fn>
dart-sdk/lib/_internal/js_dev_runtime/patch/async_patch.dart 45:50       <fn>
dart-sdk/lib/async/zone.dart 1661:54                                     runUnary
dart-sdk/lib/async/future_impl.dart 156:18                               handleValue
dart-sdk/lib/async/future_impl.dart 840:44                               handleValueCallback
dart-sdk/lib/async/future_impl.dart 869:13                               _propagateToListeners
dart-sdk/lib/async/future_impl.dart 641:5                                [_completeWithValue]
dart-sdk/lib/async/future_impl.dart 715:7                                callback
dart-sdk/lib/async/schedule_microtask.dart 40:11                         _microtaskLoop
dart-sdk/lib/async/schedule_microtask.dart 49:5                          _startMicrotaskLoop
dart-sdk/lib/_internal/js_dev_runtime/patch/async_patch.dart 181:15      <fn>

```

lib/filesharing/filesharing_logic/lib/src/models/folder.dart

@@ -138,7 +138,7 @@ class Folder {
           fileSharingData: fileSharingData,
           folderName: folderName,
           folderPath: folderPath,
-          attempt: attempt++);
+          attempt: attempt + 1);
     } else {
       return nameID;
     }