This is the very reason why i don't put my bitcoin exchange. As we can read from news, more and more cryptoexhanges have been hacked because they are the number 1 targets for hackers because hackers know there's a lot of money in there as some crypto holders and investors are putting their assets there.
IMO , any exchange include your online wallet are not the suitable place to keep your bitcoin fund.
Offline wallet , IMO , the safest one.
As we already aware when everything in internet, they can be hack , it's just the matter of time or how advance the exchange's security can improve with hacker's ability !