Yes Bitcoin can be stolen and and isn't 100% safe from hackers and this is a straight answer to your question. If you don't protect your coin in your wallet be it a Bitcoin wallet or on an exchange you are definitely going to loose it to hackers and probably fraudsters who have gotten access to your wallet by virtue of you giving them access knowingly or unknowingly, your seed phrase is a very important component of your wallet id you must be able to completely protect your wallet from access to hackers and probably fraudsters.

Talking about the safest way to invest well, I wouldn't want to rule out Bitcoin but then it's important you understand that Bitcoin was basically created for the purpose of having a decentralized currency online and not to be just invested upon but the demand and supply has affected it's price over the years.