Adding Test Set with no Tests to a Test Plan shows no error message

Description
Xray is not warning users that no Test is added to the Test Plan when the Test Set is empty.

Pre-conditions
Have an empty Test Set:

How to reproduce

1. Enter a Test Plan and go to "Add Tests" >> "Tests from Test Sets":
   
2. Select the empty Test Set:
   

Actual result/Attachments
The page refreshes, and nothing happens.

Expected result
A warning is shown, as it is when the same scenario with a Test Execution:

Version

• Xray v5.1.7 and v6.0.2