feat: Allow changing parent from null parent (flame-engine#1662)

changeParent should be allowed even when the current parent is null.

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

@@ -233,8 +233,10 @@ class Component {
   set parent(Component? newParent) {
     if (newParent == null) {
       removeFromParent();
+    } else if (_parent == null) {
+      addToParent(newParent);
     } else {
-      changeParent(newParent);
+      newParent.lifecycle._adoption.add(this);
     }
   }
 
@@ -603,7 +605,7 @@ class Component {
   /// Changes the current parent for another parent and prepares the tree under
   /// the new root.
   void changeParent(Component newParent) {
-    newParent.lifecycle._adoption.add(this);
+    parent = newParent;
   }
 
   //#endregion

packages/flame/test/components/component_lifecycle_test.dart

@@ -106,6 +106,26 @@ void main() {
       },
     );
 
+    testWithFlameGame(
+      'component mounted completes when changing parent from a null parent',
+      (game) async {
+        final parent = LifecycleComponent('parent');
+        final child = LifecycleComponent('child');
+        game.add(parent);
+
+        final mounted = child.mounted;
+        await game.ready();
+
+        child.changeParent(parent);
+        game.update(0);
+        await game.ready();
+
+        expect(child.parent, parent);
+        expect(child.isMounted, true);
+        await expectLater(mounted, completes);
+      },
+    );
+
     testWithFlameGame(
       'Component.mounted completes after the component is mounted',
       (game) async {