Mastery glitch

I was playing on the old client earlier and I attempted to make an ezreal mastery page during a normal blind pick game. After creating the masteries and clicking save i got an error. I can't remember what the error say. I couldn't select anymore mastery pages or the error would pop up. I now have no chance to change my masteries and am stuck using a blank page that i can't edit. Please fix this. I've attempted to restart my client but to no avail. I have no choice but to play with a blank mastery page.