"OK" button is blocked only for the first "External tools validation" task

Scenario:

1. Open the "Application Settings" dialog.
2. Select the "External Tools" tab.
3. Click "..." button to select a folder with all external tools.
4. Select a folder with full external tool pack.
   Expected result: a validation task is launched, "OK" button in the dialog is blocked.
5. Wait until the first validation task finishes.
   Expected result: an additional validation task is launched, "OK" button in the dialog is blocked.
   Current result: an additional validation task is launched, "OK" button in the dialog is not blocked.