This problem has been making the rounds in stackoverflow, if this issue is correct it is related with some wrong reflection that tries to load a class that is not present when a parameter is null in your code
It looks like there may be a race condition between PurchaseAsync and
the HandleActivityResult callback. tcsPurchase is being set after the
call to serviceConnection.Service.GetBuyIntent which I think allows
the HandleActivityResult call happen while tcsPurchase is still null.
When I pass an empty string for the “payload”, it gets deserialized as
null, causing the comparison to fail when checking for success.
So make sure the payload is not null before calling that method.