Forced to click greyed out button

When trying to fight in the grand arena the game is forcing me to click the battle button. I can't click that button because I have 0 toons selected and it's impossible to start a battle with an empty squad. I had this problem 15 minutes earlier in the assault battle event but my squad was autofilled there so I could start the battle, back out and pick the squad I wanted. Even with an autofilled squad that's of course not an option in grand arena because I'd lose that squad and banners.