fix: Allow null passthrough parent (#2821)

In `CollisionPassthrough`, while searching for an ancestor with
`CollisionCallbacks` mixin, `firstWhere` was used. This was causing it
to fail when no such ancestor was found. But having a passthrough parent
is not necessary, at least rest of the code wasn't designed on that
assumption. So this PR replaced `firstWhere` with `firstWhereOrNull`
allowing the passthrough to be null.

This was discovered in one of the [discord
discussions](https://discord.com/channels/509714518008528896/516639688581316629/1163600349643411467).

packages/flame/lib/src/collisions/collision_passthrough.dart

@@ -1,3 +1,4 @@
+import 'package:collection/collection.dart';
 import 'package:flame/collisions.dart';
 import 'package:flame/components.dart';
 import 'package:meta/meta.dart';
@@ -14,9 +15,9 @@ mixin CollisionPassthrough on CollisionCallbacks {
   @mustCallSuper
   void onMount() {
     super.onMount();
-    passthroughParent = ancestors().firstWhere(
+    passthroughParent = ancestors().firstWhereOrNull(
       (c) => c is CollisionCallbacks,
-    ) as CollisionCallbacks;
+    ) as CollisionCallbacks?;
   }
 
   @override

packages/flame/test/collisions/collision_passthrough_test.dart

@@ -1,5 +1,6 @@
+import 'package:flame/collisions.dart';
 import 'package:flame/components.dart';
-import 'package:flame/src/collisions/collision_passthrough.dart';
+import 'package:flame_test/flame_test.dart';
 import 'package:test/test.dart';
 
 import 'collision_test_helpers.dart';
@@ -40,5 +41,22 @@ void main() {
         expect(hitboxParent.onCollisionCounter, 1);
       },
     });
+
+    testWithFlameGame('Null passthrough', (game) async {
+      final hitbox = CompositeHitbox(children: [RectangleHitbox()]);
+      final component = PositionComponent(children: [hitbox]);
+      final testBlock = TestBlock(Vector2.zero(), Vector2.all(10));
+
+      await game.addAll([component, testBlock]);
+      await game.ready();
+
+      expect(hitbox.passthroughParent, isNull);
+
+      hitbox.removeFromParent();
+      testBlock.add(hitbox);
+      await game.ready();
+
+      expect(hitbox.passthroughParent, testBlock);
+    });
   });
 }