This is strange..!
Can you check you installed the last version correctly ? Go to the Credits screen and it should show at the top : Build 8-2018.6.12 or 6.13.
If it's not your version, download & reinstall the demo now.
Else, could you open your Tennis.ini (or the .ini of your profile if you have created one), and look for the line "[Shirt]" (without the quote) and delete, then save and launch the game, and check the t-shirt is ok or not..?