Undo after polygon-delete crashes the GUI

[codebot]

Bug report

Thanks to @samuelrawrs for finding this bug!

Required information:

• Operating system and version:
  • Ubuntu 20.04.3
• OpenRMF installation type:
  • source
• OpenRMF version or commit hash
  • main
• ROS distribution and version:
  • galactic
• ROS installation type:
  • debs
• Package or library, if applicable:
  • rmf_traffic_editor

Steps to reproduce the bug

1. Delete any floor polygon.
2. Press Ctrl-Z or click "Undo"
3. The GUI crashes