Steps to reproduce:
Navigate to dashboard
Click on edit and place new widget - Graph on that

Result:
An error message appears

Invalid parameter "Data set/1/hosts": cannot be empty.

just right after the placing the widget which can be misleading to the user ad he even didn't try save it so far. This will also happen if you will place new Data set.
Please see the screenshots attached
Expected:
The message should appear only when you have empty Data set field and are trying to save.