Didn't Receive Champion From New Mystery Champion

Once I claimed the blue essence from the hextech crafting I decided to buy five mystery champions. After just receiving Kog'maw from my fourth mystery champion I opened the last one. At the same time my friend invited me to a game and I accepted right when the champion was about to appear. The client left the loot and went into the lobby but I was curious who I received from the last mystery champion. As I checked my loot what I got, it was not there anymore. I then checked the notifications and closed the notification that I permanently unlocked Kog'Maw, but it didn't show anything after and I couldn't find any new champions in my collection. I am not upset about this but I am just wondering if this was a bug or if it was my fault for not claiming the champion before joining the lobby.