MotionPlanningFrame: Gracefully handle undefined parent widget

[rhaschke]

If the MP plugin was created in a non-rviz context (e.g. directly via librviz),
there might be no window manager and thus no parent widget available for panels.

This fixes ros-visualization/rviz#1653.

[codecov]

Codecov Report

Merging #2833 (976e22e) into master (0d38983) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2833      +/-   ##
==========================================
+ Coverage   60.80%   60.81%   +0.01%     
==========================================
  Files         366      366              
  Lines       31717    31717              
==========================================
+ Hits        19282    19284       +2     
+ Misses      12435    12433       -2     

Impacted Files	Coverage Δ
...meterization/work_space/pose_model_state_space.cpp	80.51% <0.00%> (-1.88%)	⬇️
...nning_scene_monitor/src/planning_scene_monitor.cpp	68.27% <0.00%> (+0.14%)	⬆️
...dl_kinematics_plugin/src/kdl_kinematics_plugin.cpp	78.25% <0.00%> (+0.39%)	⬆️
...on/pick_place/src/approach_and_translate_stage.cpp	74.03% <0.00%> (+0.65%)	⬆️
...ipulation/pick_place/src/manipulation_pipeline.cpp	71.30% <0.00%> (+1.86%)	⬆️

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