You'd be surprised how often we get this one. Frankly, I'm surprised how often we get this one, given that the answer is "it's not a problem".
Here's the scoop.
Holding your stocks at a custodian broker is exactly the same as holding cash at a bank - if anything, it's even safer than cash in the bank, because brokers are required to hold their clients' assets in a segregated account. (If you're trading on margin, this answer is slightly different, but let's assume you're not using any margin loans.)
If the broker goes out of business, usually what happens is that another broker buys the old broker's book of business, and everything, including your stocks and your cash, gets transferred over to the new broker.
US brokers, like Interactive, have insurance as well - so even if there's malfeasance, the insurer will cover the losses to client accounts.
If the ETF fund manager goes out of business, one of two things will happen: either the ETF will sell all its holdings, wind up, and hand you back the cash equivalent of your holdings; or the ETF management will get transferred to another fund manager, and everything will truck on as if nothing had happened.