Possible bug in rviz

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I got the following message when I try to run rviz through urdf_tutorials:

(rviz:3447): Gtk-WARNING **: Unable to locate theme engine in module_path: "pixmap",

(rviz:3447): Gtk-WARNING **: Unable to locate theme engine in module_path: "pixmap",

(rviz:3447): Gtk-WARNING **: Unable to locate theme engine in module_path: "pixmap",

(rviz:3447): Gtk-WARNING **: Unable to locate theme engine in module_path: "pixmap",
The program 'rviz' received an X Window System error.
This probably reflects a bug in the program.
The error was 'BadDrawable (invalid Pixmap or Window parameter)'.
  (Details: serial 22 error_code 9 request_code 137 minor_code 3)
  (Note to programmers: normally, X errors are reported asynchronously;
   that is, you will receive the error a while after causing it.
   To debug your program, run it with the --sync command line
   option to change this behavior. You can then get a meaningful
   backtrace from your debugger if you break on the gdk_x_error() function.)

The log says

[ INFO] [1330220460.687401287]: rviz revision number 1.6.7
[ INFO] [1330220460.687490754]: ogre_tools revision number 1.6.2
[ INFO] [1330220460.687512474]: compiled against OGRE version 1.7.3 (Cthugha)
[ INFO] [1330220460.790543031]: Loading general config from [/home/conrado/.rviz/config]
[ INFO] [1330220460.791031850]: Loading display config from [/home/conrado/.rviz/display_config]
[ INFO] [1330220460.859120679]: RTT Preferred Mode is PBuffer.

The most strange of all that is that it runs perfectly when I first run it (sometimes it works when I wait a long time before running again or lock the screen after the error but the ruslaunch command still executing). If I close and try to reopen, it gives that error. I checked and there is no program running in background that could cause some problem. I'm using Ubuntu 11.10. I'll keep searching for the source of the problem, as it seems related to the X library itself.