When setAvatarUrl is called with a NULL value (happening by default on our leaderboard, no custom calls from our end), this line throws a NULL exception error (which causes other cascading issues). It should check if avatarUrl is NULL before attempting to call avatarUrl.StartsWith().
Getting the same error while calling Social.LoadScores function. It's working fine if the user enters the native Play Games leaderboard menu before this function gets called.
https://github.com/playgameservices/play-games-plugin-for-unity/blob/d3073e3948309e23e68e39b41e919cd7f7bcf77d/Assets/Public/GooglePlayGames/com.google.play.games/Runtime/Scripts/ISocialPlatform/PlayGamesUserProfile.cs#L210
When setAvatarUrl is called with a NULL value (happening by default on our leaderboard, no custom calls from our end), this line throws a NULL exception error (which causes other cascading issues). It should check if avatarUrl is NULL before attempting to call avatarUrl.StartsWith().