First of all don't worry. The most important thing is that your bank app is protected and hopefully your device too.

Even if you left the BOPP app unprotected and someone managed to access your device, they cannot authorise payments from you bank unless they also have access to your bank through your bank app.  If you have used some of the PIN protections available within the BOPP app itself they would not be able to access BOPP either.

If your phone has been stolen or lost and you want to continue using BOPP, install the app on your new device.