Fix error message for existing labels in right sidebar.

What does this MR do?

Fixes flash notification in right sidebar, if we want to create an existing label.

Are there points in the code the reviewer needs to double check?

Nope.

Why was this MR needed?

Because it was broken.

What are the relevant issue numbers?

Fixes #20117 (closed)

Screenshots (if relevant)

Before

After

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together