refactor: modified unlock method due to invalid cast

lib/game/components/flipper.dart

@@ -193,12 +193,9 @@ class FlipperAnchorRevoluteJointDef extends RevoluteJointDef {
   /// its resting position.
   // TODO(alestiago): consider refactor once the issue is solved:
   // https://github.com/flame-engine/forge2d/issues/36
-  static void unlock(RevoluteJoint joint) {
-    assert(joint.bodyA is Flipper && joint.bodyB is Anchor, 'Invalid joint.');
-    final flipper = joint.bodyA as Flipper;
-
+  static void unlock(RevoluteJoint joint, BoardSide side) {
     late final double upperLimit, lowerLimit;
-    switch (flipper.side) {
+    switch (side) {
       case BoardSide.left:
         lowerLimit = joint.lowerLimit * -1;
         upperLimit = joint.upperLimit;

lib/game/pinball_game.dart

@@ -81,8 +81,14 @@ class PinballGame extends Forge2DGame with FlameBloc, KeyboardEvents {
     await _leftFlipper.hasMounted.future;
     await _rightFlipper.hasMounted.future;
 
-    FlipperAnchorRevoluteJointDef.unlock(_leftFlipperRevoluteJoint);
-    FlipperAnchorRevoluteJointDef.unlock(_rightFlipperRevoluteJoint);
+    FlipperAnchorRevoluteJointDef.unlock(
+      _leftFlipperRevoluteJoint,
+      BoardSide.left,
+    );
+    FlipperAnchorRevoluteJointDef.unlock(
+      _rightFlipperRevoluteJoint,
+      BoardSide.right,
+    );
   }
 
   @override