Clearing default opening app
There are multiple ways to clear the default action app:


you already noticed it gets cleared when the corresponding app is deleted
same applies if you install another app that could perform the same action
there's a menu in settings where you can clear the default without (un)installing an app. Unfortunately, it's hard to find. Before Android 7 it had an own menu entry in settings. Since Android 7, you need to go to Settings › Apps, tap the app's entry, and will find if it's set as default. With that option you can also made it the default, or remove it from being default.