tf module suddently not working anymore

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Hello,

I have been working with Gazebo and the tf libraries one day, and the next day, the tf library seems to have issues. I am not able to run Gazebo properly and load robot models and I could not even import the tf module in Python anymore.

I run ROS Melodic on Ubuntu 18.04 (actually Pop!_OS) on an Intel i5 10th gen CPU, no GPU.

I do not know, what lead to this issue.

I tried to uninstall the ros-melodic-tf package which then removed many other packages. After re-installing them, it still doesn't work.

I then downloaded the tf ROS package from the GitHub repo and added it to the catkin_ws and make a catkin_make. Now, I can import tf in Python again, but I can't run Gazebo properly nor can I use rqt_tf_tree.

When I run rosrun rqt_tf_tree rqt_tf_tree, I get the following error:

rosrun rqt_tf_tree rqt_tf_tree 
RosPluginProvider.load(rqt_tf_tree/RosTfTree) exception raised in __builtin__.__import__(rqt_tf_tree.tf_tree, [RosTfTree]):
Traceback (most recent call last):
  File "/opt/ros/melodic/lib/python2.7/dist-packages/rqt_gui/ros_plugin_provider.py", line 80, in load
    attributes['module_name'], fromlist=[attributes['class_from_class_type']], level=0)
  File "/opt/ros/melodic/lib/python2.7/dist-packages/rqt_tf_tree/tf_tree.py", line 39, in <module>
    from tf2_msgs.srv import FrameGraph
ImportError: No module named srv

PluginManager._load_plugin() could not load plugin "rqt_tf_tree/RosTfTree":
Traceback (most recent call last):
  File "/opt/ros/melodic/lib/python2.7/dist-packages/qt_gui/plugin_handler.py", line 102, in load
    self._load()
  File "/opt/ros/melodic/lib/python2.7/dist-packages/qt_gui/plugin_handler_direct.py", line 55, in _load
    self._plugin = self._plugin_provider.load(self._instance_id.plugin_id, self._context)
  File "/opt/ros/melodic/lib/python2.7/dist-packages/qt_gui/composite_plugin_provider.py", line 72, in load
    instance = plugin_provider.load(plugin_id, plugin_context)
  File "/opt/ros/melodic/lib/python2.7/dist-packages/qt_gui/composite_plugin_provider.py", line 72, in load
    instance = plugin_provider.load(plugin_id, plugin_context)
  File "/opt/ros/melodic/lib/python2.7/dist-packages/rqt_gui_py/ros_py_plugin_provider.py", line 61, in load
    return super(RosPyPluginProvider, self).load(plugin_id, plugin_context)
  File "/opt/ros/melodic/lib/python2.7/dist-packages/qt_gui/composite_plugin_provider.py", line 72, in load
    instance = plugin_provider.load(plugin_id, plugin_context)
  File "/opt/ros/melodic/lib/python2.7/dist-packages/rqt_gui/ros_plugin_provider.py", line 90, in load
    raise e
ImportError: No module named srv

Does anyone know how I can fix this issue? When I search on google for this issue, I mostly find questions about rosservices or Python modules. I can't find anything about a module called srv.

Please let me know when I need to provide more info.

With kind regards,

Dave