You can use app lock to restrict access to your app even when your system is unlocked.
Go to Settings and Tap on App lock.
Then turn on the App lock switch. You will be prompted to enter a pin or fingerprint. When it is successful App Lock will be turned on.
You can set the time out for your App Lock
You can set a pin instead of a fingerprint if you are more comfortable with it (or if your mobile is not fingerprint friendly).
You can also change or remove the pin.