Rviz error skipping XML document

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No file called /opt/ros/indigo/share/rviz_backdrop/plugin_description.xml seems to exist, [..]

And that I think is the problem.

The error message is slightly confusing - but logical - in this case: internally, the XML document is empty (as it doesn't exist), causing the parser to assume it's malformed (as no elements includes no root element).

You don't mention it, but if you installed rviz_backdrop (part of pr2_hack_the_future) from debs, then looking at the CMakeLists.txt it seems the rviz_backdrop package doesn't correctly install all its non-executable assets (ie: there is no install target for the plugin_description.xml file).

If you didn't install from debs, but ran catkin_make install on the workspace containing the pr2_hack_the_future packages, the result would be the same.

If you don't need the rviz_backdrop plugin, you can ignore this warning (the plugin will just not be found).

In any case, reporting this to the pr2_hack_the_future/issues tracker would be the best thing to do, so the maintainer can fix the build script.