In general, I think people are often too worried about this. In any industry where you already have the relationships with the customers, you are already ahead of the competition. It takes hard work to foster relationships and sell well. Any dev who thinks ripping off an idea is easy money is sorely mistaken. Without marketing and potentially funding (depending on the kind of idea), it is just code sitting on a server somewhere that nobody will ever find.
Also, as a developer who does occasional contracts to help people build out their MVPs, it would be career-suicide to steal an idea - the last thing I would want is bad word of mouth. Entrepreneurs do sometimes cluster, attend meetups, bump into each other when pitching etc.....word gets around fast in that kind of circle.
And lastly, as someone who works closely with other developers on occasion, for any given idea, half of the devs I chat to don't think the idea has legs, the other half have no inclination/time to build it themselves.
In the end, if you are hiring someone to build it for you, chat to some developers, get a feel for them, ask for a reference or two if you don't know them well (in other words, do you due diligence) and go from there.