Created attachment 747563 [details]
Application log after "delete" button is clicked

Description of problem:
When a button for deleting some data provider is clicked, nothing happens.
It seems that this is caused by the data provider being used in some KPI Panels - but the problem is that no indication is given to the user why the provider was not deleted.

Version-Release number of selected component (if applicable):

How reproducible:
Always

Steps to Reproduce:
1. Navigate to Workspace "Showcase", page "Data providers"
2. Click the button "Delete Data Provider" next to some of the out-of-the box data providers, for example "Sales dashboard demo".
  
Actual results:
Page reloads, but nothing else happens - provider is not deleted, it is left in place.

Expected results:
When user tries to delete a data provider, that cannot be deleted (e.g. because it's used in some KPI panels) some message should be displayed indicating why it is not possible to delete that provider (e.g. "It is impossible to delete this data provider, because it is used in 5 panels" or "Cannot delete data provider X, because it is used by 5 panels" or something similar).

Additional information:
When the delete button is clicked, the following error appears in the application log (see the attachment)