Problem description: in case if a dataset was removed from SVG graph widget configuration, a new dataset was added, and the color of the new dataset was changed, the following error is displayed when attempting to save widget configuration:
Invalid parameter "Data set/3/hosts": cannot be empty.

Steps to reproduce:

1. Create an SVG graph widget with 2 datasets and save it's configuration
2. Edit the created graph widget
3. Remove dataset No. 2
4. Add a new dataset
5. Change the color of the new dataset
6. Attempt to save widget configuration

Result:
The above mentioned error is observed.
Expected:
Widget configuration should be successfully saved.

Example: