Leaving it on will not cause any problems surely? Unless the unit detects a wirelessly rechargeable device it won't operate?
Looking through the owner's handbook, there doesn't appear to be any button to switch it off, only operation condition indicators on the tray side and the multimedia screen.
What it does say in the handbook (page 345) is -
"Do not use the charging tray as a small
storage space"
https://d24bc9lyrt5en5.cloudfront.net/Customer-Portal-Admin/emanuals/LEXUS/OM33G50E.pdf