Symptom

With the new 4.21.0 opt-in PlacementScene(groundShadows = true), the app crashes with a
NullPointerException the instant ARCore detects the first plane — i.e. within the first
seconds of any AR placement session on a real device.

The emulator is NOT affected (it never converges a plane in the virtual scene the same way and
runs the FL1 path), which is why the 4.21.0 emulator QA pass did not catch it.

java.lang.NullPointerException: Attempt to invoke ... on a null object reference
    at io.github.sceneview.ar.node.ShadowReceiverPlaneNode.update(ShadowReceiverPlaneNode.kt)
    at io.github.sceneview.ar.node.PlaneNode.<init>(PlaneNode.kt)        // init { trackable = plane }
    at io.github.sceneview.ar.node.ShadowReceiverPlaneNode.<init>(ShadowReceiverPlaneNode.kt)
    ...

Root cause

The base PlaneNode's init { trackable = plane } invokes the open update(trackable) —
virtually dispatched to ShadowReceiverPlaneNode's override — before the subclass's
meshNode field is initialized, so the override dereferences a null meshNode and crashes
on the first detected plane. Classic leaking-this-in-constructor virtual dispatch.

Affected versions

• 4.21.0 only — groundShadows was introduced in 4.21.0 (Sprint 1 — V2 redo: PlaneDiscoveryGuide + ShadowReceiverPlane + PlacementReticle + ARPlacementDemo modernisée #2241). Earlier releases do not
  have the option; the default groundShadows = false is safe.

Workaround (on 4.21.0)

PlacementScene(
    groundShadows = false, // default — avoids the crash, loses contact shadows
    ...
)

Fix

Fixed on main by 30de607
(fix(arsceneview): NPE crash on plane detection with groundShadows): update() now no-ops
until construction finishes (constructed flag), and the receiver mirrors PlaneVisualizer's
device-proven flat-receiver recipe (culling(false) + non-degenerate AABB). Regression
contract tests added (ShadowReceiverPlaneContractTest).

Ships in 4.21.1. The v4.21.0 release notes carry a "Known issue" advisory pointing here.

Discovered while root-causing the AR device-QA leg failure (#2620) from a real Pixel 9 stack trace.