Error in Understanding ROS Topics Tutorial - rosrun rqt_graph rqt_graph error

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I am following the tutorial http://wiki.ros.org/ROS/Tutorials/Und...

I am using Ubuntu 20.04, and am using ROS noetic.

I have followed the tutorial step by step and after I run the command:

rosrun rqt_graph rqt_graph

I run into the following error:

PluginManager._discover() force discovery of plugins
RospkgPluginProvider._find_plugins() crawling for plugins of type 'qt_gui'
CompositePluginProvider.discover() could not discover plugins from provider "<class 'rqt_gui.rospkg_plugin_provider.RospkgPluginProvider'>":
Traceback (most recent call last):
  File "/opt/ros/noetic/lib/python3/dist-packages/qt_gui/composite_plugin_provider.py", line 57, in discover
    plugin_descriptors = plugin_provider.discover(discovery_data)
  File "/opt/ros/noetic/lib/python3/dist-packages/rqt_gui/ros_plugin_provider.py", line 67, in discover
    plugin_descriptors += self._parse_plugin_xml(package_name, plugin_xml)
  File "/opt/ros/noetic/lib/python3/dist-packages/rqt_gui/ros_plugin_provider.py", line 128, in _parse_plugin_xml
    for library_el in root.getiterator('library'):
AttributeError: 'ElementTree' object has no attribute 'getiterator'

CompositePluginProvider.discover() could not discover plugins from provider "<class 'qt_gui.recursive_plugin_provider.RecursivePluginProvider'>":
Traceback (most recent call last):
  File "/opt/ros/noetic/lib/python3/dist-packages/qt_gui/composite_plugin_provider.py", line 57, in discover
    plugin_descriptors = plugin_provider.discover(discovery_data)
  File "/opt/ros/noetic/lib/python3/dist-packages/qt_gui/recursive_plugin_provider.py", line 53, in discover
    plugin_descriptors = self._plugin_provider.discover(discovery_data)
  File "/opt/ros/noetic/lib/python3/dist-packages/rqt_gui/ros_plugin_provider.py", line 67, in discover
    plugin_descriptors += self._parse_plugin_xml(package_name, plugin_xml)
  File "/opt/ros/noetic/lib/python3/dist-packages/rqt_gui/ros_plugin_provider.py", line 128, in _parse_plugin_xml
    for library_el in root.getiterator('library'):
AttributeError: 'ElementTree' object has no attribute 'getiterator'

RospkgPluginProvider._find_plugins() crawling for plugins of type 'rqt_gui'
CompositePluginProvider.discover() could not discover plugins from provider "<class 'qt_gui.recursive_plugin_provider.RecursivePluginProvider'>":
Traceback (most recent call last):
  File "/opt/ros/noetic/lib/python3/dist-packages/qt_gui/composite_plugin_provider.py", line 57, in discover
    plugin_descriptors = plugin_provider.discover(discovery_data)
  File "/opt/ros/noetic/lib/python3/dist-packages/qt_gui/recursive_plugin_provider.py", line 53, in discover
    plugin_descriptors = self._plugin_provider.discover(discovery_data)
  File "/opt/ros/noetic/lib/python3/dist-packages/rqt_gui/ros_plugin_provider.py", line 67, in discover
    plugin_descriptors += self._parse_plugin_xml(package_name, plugin_xml)
  File "/opt/ros/noetic/lib/python3/dist-packages/rqt_gui/ros_plugin_provider.py", line 128, in _parse_plugin_xml
    for library_el in root.getiterator('library'):
AttributeError: 'ElementTree' object has no attribute 'getiterator'

qt_gui_main() found no plugin matching "rqt_graph.ros_graph.RosGraph"
try passing the option "--force-discover"

In a different posts listed below:

https://github.com/ros-visualization/... https://answers.ros.org/question/3665...

the accepted solution: is that I change getiterator to iter in /opt/ros/noetic/lib/python3/dist-packages/rqt_gui/ros_plugin_provider.py. I can locate this file however my file is read-only!

What can I do? I can not find any solutions to this.