Disable trade UI elements when leagues are not populated

[Wires77]

Fixes #8952 .

Description of the problem being solved:

This pane would show an error when leagues weren't properly fetched, but would still allow users to find the best item on trade. There might be more we can do here to make it clear what the issue is, but removing the possibility of error for now seemed best.

Steps taken to verify a working solution:

• Made a typo in the trade URL to force a 404 error
• Noticed the 404 error appearing in the proper error location (could still use a UI cleanup here, but is unrelated to the core issue)
• Noticed the "Find Best" buttons were disabled