Until now nobody can hack bitcoin wallet except due to errors and inability of wallet users, we never heard blockchain on hack, it proves that this system is very safe and can be used for safe and convenient transaction
Sorry to burst your bubble but exchanges have been hacked and much money has been lost because of that, I honestly do not remember if an online wallet has been hacked but I will not be surprised if it was, that is why it is recommended in the forum that you never keep an important amount of coins in an online wallet and that instead you keep most of your coins in a wallet you control and if possible that you keep that wallet in cold storage.