Here's a hot take (not really): an idea on its own isn't worth squat. What does matter, is its execution. So why worry about other people potentially stealing your precious idea?
Sure it's normal to be protective over anything that could help the company grow, but let's face it: good ideas are easy to come by. Go to literally any bar past midnight and the revelations will come flooding in.
What's worse, is that by holding off showing anything of your project that's in development to the public until the very end, it could potentially ruin you.
Why? Because if you haven't tested anything with your actual target audience beforehand, how will you know you're building the right thing? This is probably the very core of a design sprint; getting something out the door fast with the sole purpose of validating a product concept or idea.
And then there's a whole argument to be made for customer-centric design and all that, but we'll save that for another time.