Error: XR Input Action State giving NullPointerException

[rainysea]

This works for me.I updated the xr-interaction pacakge from 2.0.1 to 2.5.4 and this issue occured.

[steve186241]

@jdh5259

Running into a similar issue using:

Unity 2022.3.9f1
XR Interaction Toolkit 2.5.1
Input System 1.7.0

For us, the culprit is a bad null check in ActionBasedController.UpdateInput method. The controller was configured such that the UI Scroll Action was not using a reference and it didn’t have any action configured. However, in this case, InputActionProperty.action will return an instance of InputAction that is empty, not null. The ActionBasedController only checks if the action is null, not if the action is empty. This eventually results in a null reference when calling InputAction.ReadValue then InputActionState.ApplyProcessors.

var uiScrollAction = m_UIScrollAction.action;
if (uiScrollAction != null) <- Bad check
    controllerState.uiScrollValue = uiScrollAction.ReadValue<Vector2>();

As a workaround until the code is fixed, we checked the “Use Reference” box for the UI Scroll Action and simply left the reference null.

Hopefully that helps.

EDIT: Also encountered a similar issue with XRRayInteractor and the translate/rotate anchor inputs. A similar check against null causes the same error.

static bool TryRead2DAxis(InputAction action, out Vector2 output)
{
    if (action != null) <- Bad check
    {
        output = action.ReadValue<Vector2>();
        return true;
    }
    output = default;
    return false;
}

Thank you! This was exactly my problem. Checking Use Reference fixed the issue! For context, I was using Unity 2021.3 and XR Toolkit 2.5.4. This bug likely occurs for people that are on XR Toolkit upgrade paths or that create custom ActionBasedControllers. The Unity XR Starter Kit has these references populated in their prefabs, so it won’t be an issue there.

I checked the change logs in the XR Toolkit and it doesn’t appear that Unity has fixed this issue yet in any package.

Here is an explanation of why this occurs:
If the UI Scroll Action is not assigned, InputActionProperty.action will return an instance of InputAction that is effectively empty, not null. This means the null check (if (uiScrollAction != null)) is insufficient, as it doesn’t handle the case where the action exists but is unbound or has no meaningful configuration.

The problem is that even though the action instance exists, attempting to call methods like ReadValue() on an unbound or invalid InputAction can result in runtime errors. Specifically, InputActionState.ApplyProcessors may throw a NullReferenceException because there are no bindings or valid state associated with the action.

This bug isn’t present everywhere. For example, IsTrackedAction handles the null and empty check correctly.

            // Actions without bindings are considered empty and will fallback
            if (isTrackedInputAction != null && isTrackedInputAction.bindings.Count > 0)

However, in other areas the binding check is configured incorrectly.

            var uiPressValueAction = m_UIPressActionValue.action;
            if (uiPressValueAction == null || uiPressValueAction.bindings.Count <= 0)
                uiPressValueAction = m_UIPressAction.action;
            controllerState.uiPressInteractionState.SetFrameState(IsPressed(m_UIPressAction.action), ReadValue(uiPressValueAction));

I recommend checking UseReference for all actions and leave them empty if you don’t use them.

Hopefully Unity fixes this issue at some point.