Workflow can't be saved, or `null` as a content

### `null` in Workflow json instead of export

### Expected Behavior

Save workflow as json

### Actual Behavior

Saves json with `null`. Lost everything

### Steps to Reproduce

* I created subgraph with custom nodes
* Saved, and i lost all workflows

### Debug Logs

```powershell
Have no idea. it has no logs files in the directory of the app

Only got message that it can't write something into null.
```

### Other