Fix rviz crash if launch later

[mini-1235]

---

Basic Info

Info	Please fill out this column
Ticket(s) this addresses	#4754
Primary OS tested on	Ubuntu 24.04
Robotic platform tested on	turtlebot 3 simulation
Does this PR contain AI generated software?	No

---

Description of contribution in a few bullet points

Revival of #4871 targeting the main branch with improved code

• Open a new thread to load plugins, to ensure that get_parameter() would not block the main thread. The combo box will now have value if it successfully gets the parameter, otherwise empty

Description of documentation updates required from your changes

None

Description of how this change was tested

Extensively tested on tb3 simulation and all tests passed

---

Future work that may be required in bullet points

For Maintainers:

• Check that any new parameters added are updated in docs.nav2.org
• Check that any significant change is added to the migration guide
• Check that any new features OR changes to existing behaviors are reflected in the tuning guide
• Check that any new functions have Doxygen added
• Check that any new features have test coverage
• Check that any new plugins is added to the plugins page
• If BT Node, Additionally: add to BT's XML index of nodes for groot, BT package's readme table, and BT library lists

[mini-1235]

I now add the combo_box->addItem("Default"); after we have loaded the parameter successfully, but I can move it before loading the plugin if you think it is better

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

see 6 files with indirect coverage changes

[SteveMacenski]

Otherwise, small thing to update and should be good to go!