fix: Allow setting bounds while BoundedPositionBehavior's target …

…is null (#2926)

The `bounds` setter of `BoundedPositionBehavior` tries to update the
target's position when bounds are updated. But it wasn't checking if the
target is null. This was causing null exceptions while updating bounds
of an unmounted `BoundedPositionBehavior` with null target (as seen in
[this failing test
case](https://github.com/flame-engine/flame/actions/runs/7231182930/job/19704091006#step:5:1291)).
This PR fixes that by checking if the target is null before updating the
position.


Closes #2655

packages/flame/lib/src/camera/behaviors/bounded_position_behavior.dart

@@ -36,7 +36,9 @@ class BoundedPositionBehavior extends Component {
     _bounds = newBounds;
     if (!isValidPoint(_previousPosition)) {
       _previousPosition.setFrom(_bounds.center);
-      update(0);
+      if (_target != null) {
+        update(0);
+      }
     }
   }
 

packages/flame/test/camera/behaviors/bounded_position_behavior_test.dart

@@ -38,6 +38,13 @@ void main() {
       );
     });
 
+    test('update bounds while target is null', () {
+      final circle1 = Circle(Vector2.zero(), 10);
+      final circle2 = Circle(Vector2.all(30), 10);
+      final boundedPositionBehavior = BoundedPositionBehavior(bounds: circle1);
+      expect(() => boundedPositionBehavior.bounds = circle2, returnsNormally);
+    });
+
     testWithFlameGame('bad parent', (game) async {
       final shape = Circle(Vector2.zero(), 10);
       final parent = Component()..addToParent(game);