This is a known issue if you delete the definition of your default Data Source. You can check the ID/Name of your default data source through Control Panel -> Framework Settings -> df.defaultDataSource. The default name is .. "default".
You don't need to use the default data source in your application, but you do need to have a valid one defined for now. So deploy a default.json file (it can point to any valid data source, including the one you are using) and restart or refresh your data sources.