Well, in your example as presently stated, you did not actually make it into a "general sentence" the way I think of one. You instead made it into a question, which you did refer to earlier, so I will go with the impression that an idiomatic question is your goal.
If so, I would correct "Is there really such thing as a totally risk-free industry?" into:
Is there really such a thing as a totally risk-free industry?
To be honest, I do not know why changing from a statement to a question changes the idiom in this way. I recognize that, "There really is no such a thing as a totally risk-free industry" does not sound truly correct ... it sounds awkward and stilted to me. I can only advise that your Original Post question requires an "a" to sound like a "correct" English question (at least in the U.S.).
If "idiomatic" here means "like a native speaker," the above covers that.
But if "idiomatic" here means "I want to be sure to use a highly common idiom here," I do not think this could become more idiomatic (use of more idioms) -- though someone more creative than I may come up with a way -- but one could easily make it less idiomatic (subtracting idioms) by simplifying the sentence to:
Is there a totally risk-free industry?
or (perhaps sounding more natural)
Does a totally risk-free industry [actually] exist?
Note that the "actually" above is a completely optional emphasis.
And since I just showed how it could be made less idiomatic (avoiding idioms), I consider that proof that the (corrected) original is already idiomatic (using an idiom) as it stands, if building in a specific idiom is important to you here.
EDIT: The above "idiom" segment apparently demonstrates my own lack of awareness that non-native English speakers sometimes/often(?) use "idiomatic" to mean "like a native speaker" vs. my very literal interpretation based on the concept of an "idiom." I had to run into that usage several more times before I realized it.