If you bought gold you wanted an app selling it in case it's going to be devalued compared with € or $?
You can't freeze anything, the market is simply buyers and sellers making a deal, who did the best deal only the future will know.
You bought some cryptocurrency and you have that amount of the currency, it's yours so you can use it any time, finish. Fiat money is devaluating everyday lowering your purchase power but you are not asking for an app changing your money to products or services to avoid it devaluating too much (look at Turkish Lira).
If you do any kind of investment you should have some basic understanding of how markets work, also on the big Stock Exchanges there are days with extremely big losses in some titles.
There are stop loss orders on any exchange and you can change crypto to stable coins, that's it.
RE: The Crypto world needs a app that protects assets when the market is down