Substitutions do not work with `RegisterEventHandler`?

Bug report

Required Info:

• Operating System:
  • Ubuntu 20.04
• Installation type:
  • package
• Version or commit hash:
  • 0.10.5-1focal.20210423.000156
• DDS implementation:
  • default
• Client library (if applicable):
  • N/A

Steps to reproduce issue

cd $ROS_WORKSPACE
git clone https://github.com/galou/launch_bug.git src/launch_bug
colcon build
ros2 launch launch_bug delayed_launch.launch.py

Expected behavior

empty.launch.py should be launched after 1s.

Actual behavior

[INFO] [pause_node-1]: process started with pid [134112]
[INFO] [pause_node-1]: process has finished cleanly [pid 134112]
Task exception was never retrieved
future: <Task finished name='Task-7' coro=<LaunchService._process_one_event() done, defined at /opt/ros/foxy/lib/python3.8/site-packages/launch/launch_service.py:274> exception=SubstitutionFailure('ThisLaunchFileDir used outside of a launch file (in a script)')>
Traceback (most recent call last):
  File "/opt/ros/foxy/lib/python3.8/site-packages/launch/launch_service.py", line 276, in _process_one_event
    await self.__process_event(next_event)
  File "/opt/ros/foxy/lib/python3.8/site-packages/launch/launch_service.py", line 296, in __process_event
    visit_all_entities_and_collect_futures(entity, self.__context))
  File "/opt/ros/foxy/lib/python3.8/site-packages/launch/utilities/visit_all_entities_and_collect_futures_impl.py", line 38, in visit_all_entities_and_collect_futures
    sub_entities = entity.visit(context)
  File "/opt/ros/foxy/lib/python3.8/site-packages/launch/action.py", line 108, in visit
    return self.execute(context)
  File "/opt/ros/foxy/lib/python3.8/site-packages/launch/actions/include_launch_description.py", line 125, in execute
    launch_description = self.__launch_description_source.get_launch_description(context)
  File "/opt/ros/foxy/lib/python3.8/site-packages/launch/launch_description_source.py", line 81, in get_launch_description
    perform_substitutions(context, self.__location)
  File "/opt/ros/foxy/lib/python3.8/site-packages/launch/utilities/perform_substitutions_impl.py", line 26, in perform_substitutions
    return ''.join([context.perform_substitution(sub) for sub in subs])
  File "/opt/ros/foxy/lib/python3.8/site-packages/launch/utilities/perform_substitutions_impl.py", line 26, in <listcomp>
    return ''.join([context.perform_substitution(sub) for sub in subs])
  File "/opt/ros/foxy/lib/python3.8/site-packages/launch/launch_context.py", line 197, in perform_substitution
    return substitution.perform(self)
  File "/opt/ros/foxy/lib/python3.8/site-packages/launch/substitutions/this_launch_file_dir.py", line 56, in perform
    raise SubstitutionFailure(
launch.substitutions.substitution_failure.SubstitutionFailure: ThisLaunchFileDir used outside of a launch file (in a script)

Additional information

Defining the launch description without event handler as:

    launch_description = LaunchDescription(
        [
            pause_node,
            included_launch_description,
        ],
    )

and removing the event handler works.

Start with the reproducible launch_bug example, especially delayed_launch.launch.py and empty.launch.py, and run the provided colcon build and ros2 launch commands. Trace how RegisterEventHandler handles IncludeLaunchDescription and the ThisLaunchFileDir substitution; done means empty.launch.py launches after 1 second without the reported SubstitutionFailure.