fix: apply flat world settings to house model

src/nav2_turtlebot_simulator/launch/simulator.launch.py

@@ -52,7 +52,10 @@ def create_flat_house_world(gazebo_dir):
     os.makedirs(flat_model_dir, exist_ok=True)
 
     house_model_tree = ET.parse(model_path)
-    house_model = house_model_tree.getroot()
+    house_sdf = house_model_tree.getroot()
+    house_model = house_sdf.find('model')
+    if house_model is None:
+        raise RuntimeError('turtlebot3_house model is missing from model.sdf')
     house_model.attrib['name'] = 'turtlebot3_house_flat'
     house_static = house_model.find('static')
     if house_static is None:
@@ -90,7 +93,7 @@ def create_flat_house_world(gazebo_dir):
         lighting.text = 'false'
 
     # ElementTree serializes Gazebo's frame='' as frame="", which is invalid SDF.
-    for element in house_model.iter():
+    for element in house_sdf.iter():
         if element.attrib.get('frame') == '':
             del element.attrib['frame']