Fixes potential segfault

[arjo129]

The fix is similar in nature to #78. Just noticed another place where we are not checking for a potential null return value. This hasn't segfaulted for me yet but theres no reason why it would not.

Signed-off-by: Arjo Chakravarty arjo@openrobotics.org

[codecov]

Codecov Report

Merging #79 (1480bd7) into main (d3e0c0c) will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##            main     #79   +/-   ##
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  Coverage   0.00%   0.00%           
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  Files        144     144           
  Lines      13412   13424   +12     
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- Misses     13412   13424   +12     

Flag	Coverage Δ
tests	0.00% <ø> (ø)

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Impacted Files	Coverage Δ
...ation/rmf_robot_sim_common/src/readonly_common.cpp
...ion/rmf_building_sim_ignition_plugins/src/lift.cpp
...uilding_sim_gazebo_plugins/src/toggle_charging.cpp
...ation/rmf_robot_sim_common/src/readonly_common.cpp
...uilding_sim_gazebo_plugins/src/crowd_simulator.cpp
...on/include/rmf_robot_sim_common/slotcar_common.hpp
...ing_sim_gazebo_plugins/src/thumbnail_generator.cpp
...ion/rmf_building_sim_ignition_plugins/src/door.cpp
...nition_plugins/src/toggle_floors/toggle_floors.cpp
...lation/rmf_building_sim_common/src/lift_common.cpp
... and 262 more