To reproduce, attach with isql to the any database and run:

set term ^;
create or alter trigger trg_tx_start on transaction start
as
declare tx int;
begin
  execute statement ('select current_transaction from rdb$database')
    into :tx;
end
^
set term ;^
commit;

-- new transaction using API, OK
select current_transaction from rdb$database;
commit;

-- new transaction using SQL, AV on commit
set transaction;
select current_transaction from rdb$database;
commit;

drop trigger trg_tx_start;
commit;