Fix crash if no window_context exists

[simonschmeisser]

#491 introduced a regression:

In TrajectoryVisualization::onInitialize the existance of a window_context is checked and the trajectory_slider_panel_ and trajectory_slider_dock_panel_ are only created if it exists.
However later on these two pointers are assumed to be valid (non-NULL) and not checked any further.

[v4hn]

hey @simonschmeisser,

did you encounter this segfault or is this patch preventive?
I believe @Jntzko implemented this similar to other parts of the moveit plugins.
I'm waiting for his response first.

If this is relevant, we have to cherry-pick it to i/j too.

[simonschmeisser]

@v4hn we do encounter a crash when it tries to set the title of the non-existent panel. We use several rviz::Display and one rviz::PropertyTreeWithHelp in our application but don't have a central rviz window and thus the window_context == nullptr

[v4hn]

I see. Makes sense.
Thanks a lot for the patch and I'm sorry for the inconvenience. :/

+1 . Should be picked to i/j.

[v4hn]

@davetcoleman, @130s please review/merge.
@Jntzko also found a similar situation in the motion planning rviz plugin and will probably create a similar pull request for it later today.

[Jntzko]

The related pull request is #525.

[130s]

I'm not sure how to reproduce the issue. Assuming from the comment #523 (comment) by @simonschmeisser, it can only be replicated from source code (I don't know on RViz GUI how to set a title on a non-existent panel)? So I leave the decision up to others.

[v4hn]

merging. We want to have this regression-fix in the next release.

[v4hn]

cherry-picked to i/j