Found a bug, when I've enabled the UI, then put in a trading system module. I can't open the trading system module UI. Disabling the UI also removes the trading system UI button. It can be fixed by disabling the UI, removing the trade-module, re-adding the trade module, and then re-enabling the UI (moving the button back to the far left of the list).
Hope that helps you squash the bug.
-KineticNerd