fix(android): guard onDetachedFromEngine against null channel (NPE on headless engine teardown)

[Ant1Miller]

Problem

Reported in ZD #6122 (SuperLyfe / MAHHP), reproducible on Android and confirmed against source.

MethodChannel channel is instantiated in onAttachedToActivity (line 364), not in onAttachedToEngine (which only stores the BinaryMessenger). onDetachedFromEngine then called channel.setMethodCallHandler(null) unconditionally.

A Flutter engine that registers the plugin but never attaches an Activity — the everyday case being an FCM background isolate, but also WorkManager / background-locator engines — reaches onDetachedFromEngine with channel == null and crashes:

java.lang.NullPointerException: Attempt to invoke virtual method
'void io.flutter.plugin.common.MethodChannel.setMethodCallHandler(...)' on a null object reference
    at co.tryterra.terra_flutter_bridge.TerraFlutterPlugin.onDetachedFromEngine(TerraFlutterPlugin.java:482)

Present on 0.9.4 (this branch) and reported identical on 0.8.3.

Fix

Null-guard the teardown and clear the field:

if (channel != null) {
  channel.setMethodCallHandler(null);
  channel = null;
}

• Activity-attached teardown is unchanged (channel is non-null, handler still detaches).
• Headless teardown skips the deref instead of crashing.
• Nulling the field makes a double-detach idempotent and lets the old channel GC.

Verification

No Android unit-test harness exists in this package (src/test is empty), so this is verified by inspection rather than a runtime test: line 482 is the exact frame in the customer's crash trace, and the guard makes that dereference safe on the null-channel path while leaving the Activity-attached path untouched. Adversarial review confirmed no use-after-null on any other path (channel is read only here, written only in onAttachedToActivity).

Scope / risk

Additive defensive guard on a path that previously always crashed for headless engines — no behaviour change for Activity-attached apps. Risk to other customers: none.

Deliberately out of scope (the customer's secondary observations, to be handled separately): the success: true masking in init callbacks, the empty onDetachedFromActivity (handler leak on activity re-attach), and the getAthlete notImplemented branch.

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