Why is my phone's on screen keyboard still coming up when I use a Bluetooth keyboard?

Fern paired her Bluetooth keyboard to her mobile phone, but it keeps trying to bring up the on screen keyboard. Leo recommends re-pairing the keyboard. Go to Settings > Bluetooth, the find the keyboard and "forget" it. Then reacquire it like the first time.

The chatroom says that there's a hack to get around it. There's an app in the Google Play Store called the Null Keyboard, that will make the keyboard disappear by assigning an empty keyboard to it. But she'll need to remember that when she isn't using the Bluetooth keyboard.

There's also this workaround by GuidingTech.com.