Null reference exception while battle

This largely depends on your game's setup - that tutorial is part of a series that needs to be followed step by step. At that part of the series, there's no enemy or battle system added yet, so you can just cause the actions via the control maps as it's set up there.

However, if you add this to an existing project without e.g. having a Real Time Battle Area added to your scene (comes later in this tutorial series), running into an enemy will most likely cause a different battle system to be used (i.e. the Default Battle System that's et up in Battles > Battle System > General Settings).

Ultimately, the used battle system has set up if and which battle menu is used.

I think this is battle menu related - e.g. if you didn't set up a battle menu but have a setup that displays a target selection for an action, that could cause an error.

Based on the error, this happens when a control map is used to perform an action.

Yes - the ability's target selection settings mostly. They define what targeting is used for the action. E.g. the 3D action RPG uses None targeting, so you can just swing around your weapon without having to select a target.

Your setup most likely uses a single/group target selection, i.e. using the action (also via control maps) will trigger a target selection, which uses the battle menu. If the battle menu isn't set up properly, this can cause errors.
The control map keys have settings to automatically select targets, though, or not using target selections, etc.

Ultimately, this comes down to how your system should work - e.g. do you want just untargeted attacks where you swing around your weapon, or something like tab-targeting, etc.