Fix actor worlds

[adlarkin]

Signed-off-by: Ashton Larkin ashton@openrobotics.org

🦟 Bug fix

Fixes #647

Summary

While working on fixing the crash from launching actor.sdf (see #647), I noticed that there are issues with other actor example worlds as well. It seems like there is an issue using an <include> tag for actors in SDF (removing actor-related <include>s from the SDF resolves crashes on launch). I'm not sure when this became a problem, or why it is a problem. I need to look into this further (I haven't been involved with actor-related development for Ignition), so I've added a list of things I plan to investigate below.

TODO:

• address empty actors_population.sdf file (should we delete this, or should we populate it with the corresponding erb file?)
• Figure out when <include> tags became an issue (I need to look through the history/past PRs. One PR that I found so far that may be related is More actor components and follow plugin #157)
• Look at the stack trace in Ignition Gazebo crashes when running the example actor.sdf world #647 to see if there is any useful information there
• Update the tests for actors to handle <include> scenarios if this test case hasn't been handled yet

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge

[adlarkin]

Closing this PR because #754 seems to be the appropriate solution.