XR application crashing due to null reference exceptions

Null Reference Exceptions (NREs) are one of the most common runtime errors encountered in software development—and XR (Extended Reality) applications are no exception. When left unchecked, these errors can crash an XR app, leading to a frustrating and immersive-breaking experience for users.

This guide dives into why null reference exceptions happen, why they’re particularly problematic in XR development, and how you can prevent and fix them.

What is a Null Reference Exception?

A null reference exception occurs when your code tries to access a property or method on an object that hasn’t been initialized (i.e., it’s null). In programming languages like C#, which is commonly used in Unity (a popular engine for XR), trying to dereference a null object causes a runtime crash.

Example (C#):

GameObject player;
player.name = "XR_Player"; // This will throw a NullReferenceException because 'player' is null

Why It’s a Big Problem in XR

In XR applications, null reference exceptions can be more critical than in traditional apps because:

They often lead to hard crashes or frozen scenes rather than just throwing a visible error.
Users are fully immersed, and sudden app crashes can be disorienting or even cause motion sickness.
Complex interactions between scripts, sensors, user input, and hardware can increase the likelihood of uninitialized references.

Common Causes of Null Reference Exceptions in XR Apps

1. Unassigned References in the Inspector (Unity-specific)

Developers often forget to assign required references like controllers, UI panels, or camera rigs in Unity’s Inspector panel.

Example: A script referencing the XR rig’s left-hand controller might not work if the GameObject is left unassigned.

2. Script Execution Order

In Unity or Unreal, the order in which scripts run can lead to one script trying to access another that hasn’t yet initialized.

Example: A Start() method in Script A tries to access a GameObject in Script B that hasn’t run Awake() yet.

3. Race Conditions in Asynchronous Initialization

When XR apps load objects asynchronously (common in dynamic AR/VR scenes), code may try to access objects before they’ve been fully instantiated.

Example: A virtual object fetched from a cloud anchor might be accessed before the download completes.

4. Hardware or SDK API Failures

XR SDKs (e.g., OpenXR, ARCore, ARKit, Oculus SDK) may fail to return expected objects due to hardware limitations or permissions not being granted.

Example: If the headset’s camera fails to initialize and returns null, referencing the camera stream will crash the app.

5. Dynamic Object Destruction

XR scenes often load and unload content dynamically. If an object is destroyed but still referenced elsewhere, it leads to null references.

Symptoms of a Null Reference Crash

Sudden application exit or freeze.
Debug console logs showing something like: NullReferenceException: Object reference not set to an instance of an object
XR experience becomes unresponsive after an interaction.
Haptics or visuals break due to a controller or object reference being lost.

How to Prevent and Handle Null Reference Exceptions

✅ 1. Initialize All Variables

Always ensure variables are initialized before using them.

GameObject player = GameObject.Find("Player");
if (player != null)
{
    player.name = "XR_Player";
}

✅ 2. Use Null Checks Before Access

Protect critical object accesses using if statements or the null-conditional operator (?.).

xrController?.SendHapticImpulse(0.5f, 0.2f);

✅ 3. Validate Inspector Assignments

In Unity, use [SerializeField] and add a custom OnValidate() method to detect unassigned references during design time.

[SerializeField] private XRController leftController;

private void OnValidate()
{
    if (leftController == null)
        Debug.LogWarning("Left Controller is not assigned!");
}

✅ 4. Log Initialization Paths

Add debug logs to confirm object creation and initialization order.

void Awake()
{
    Debug.Log("Initializing hand model...");
}

✅ 5. Use Try-Catch for SDK/API Calls

Wrap external SDK interactions with proper error handling.

try
{
    var anchor = ARAnchorManager.AttachAnchor(anchorTransform);
}
catch (Exception e)
{
    Debug.LogError("Failed to attach anchor: " + e.Message);
}

✅ 6. Use Assertions or Custom Validation Utilities

Especially in development builds, use assertions to catch potential nulls before runtime.

Debug.Assert(xrCamera != null, "XR Camera reference is null!");

Tools and Practices for Debugging

Unity Debug Console: Check the stack trace to locate the script and line number of the NRE.
Unity Profiler: Use it to trace execution order or see if certain objects are being destroyed or unloaded prematurely.
Visual Studio Debugger: Set breakpoints and use watch variables to see what’s going null.
Editor Logs (Unity): Found in AppData/LocalLow/[Company]/[App]/output_log.txt.

Special Considerations for XR Devices

Device-Specific Timing: XR platforms like Meta Quest or HoloLens may delay initializing components. Make sure your code waits until systems are ready.
User Permissions: AR features like camera or spatial mapping may return null if the user hasn’t granted permissions.
Cross-Platform Differences: What works fine on one XR device might crash on another due to missing SDK support or capabilities.