fix: Check for removing state while adding a child (#3050)

Adding a child component to a parent which is in `removing` state,
caused the lifecycle processing to go into a cyclic dependency when the
parent is re-added. It happens because, while processing the lifecycle
events, child's add event causes itself and the parent to get added to
the blocked list. As a result of this, when the parent's add event is
processed next, it gets skipped due to being in the blocked list.

This PR makes sure that the child does not get enqueued when parent is
about to be removed.

packages/flame/lib/src/components/core/component.dart

@@ -594,7 +594,7 @@ class Component {
         _clearRemovingBit();
       }
       game.enqueueMove(child, this);
-    } else if (isMounted && !child.isMounted) {
+    } else if (isMounted && !isRemoving && !child.isMounted) {
       child._parent = this;
       game.enqueueAdd(child, this);
     } else {

packages/flame/test/components/component_test.dart

@@ -556,6 +556,27 @@ void main() {
           expect(wrapper.contains(child), true);
         },
       );
+
+      testWithFlameGame('when parent is in removing state', (game) async {
+        final parent = Component();
+        final child = Component();
+
+        await game.add(parent);
+        await game.ready();
+
+        // Remove the parent and add the child in the same tick.
+        parent.removeFromParent();
+        await parent.add(child);
+
+        // Timeout is added because processLifecycleEvents of ComponentTreeRoot
+        // gets blocked in such cases.
+        expect(game.ready().timeout(const Duration(seconds: 2)), completes);
+
+        // Adding the parent again should eventually mount the child as well.
+        await game.add(parent);
+        await game.ready();
+        expect(child.isMounted, true);
+      });
     });
 
     group('Removing components', () {