The easiest way to debug WebView is to connect your Android device to PC by USB and inspect your WebView by Chrome dev tools.
So, you will need:
1) Activate USB debugging on our devise. You can find it Settings >> Developer Options >> Debugging >> USB Debugging (activate checkbox)
2) Connect you devise to Computer by USB
Note: If you are developing on Windows, install the appropriate USB driver for your device. See OEM USB Drivers on the Android Developers’ site.
3) Open Chrome browser and type in the URL field: chrome://inspect/#devices
4) Confirm that ‘Discover USB devices’ activated
5) On your device, an alert prompts you to allow USB debugging from your computer. Tap OK.
6) On the chrome://inspect page displays every connected device. Click inspect for connected device and you will get console.
More detailed manual is Debugging Android WebView