Open sourced doesn`t to 100% safety and doesn`t mean it must be more secure than a wallet that not open-sourced. Because open source means that more people can act as monitors to ur code, but also gives ppl chance to figure out the mistake in your code.

Okay, back to your require. My recommendation is the Bither wallet.
Bither is an open source wallet that supports Bitcoin SegWit address. Also, a wallet that recommends by Bitcoin.org
The Bither / Bitpie dev team has a very good reputation in China and their users never lost any coins in the past 5 years according to their announcement.

Bither/Bitpie: www.bitpie.com
Bitcoin.org: https://bitcoin.org/en/choose-your-wallet