Errors when using rosbag

Error: Unindexed bag

If you see this message:

$ rxbag my_logfile.bag.active 
Error loading [my_logfile.bag.active]: Unindexed bag

or this output from rqt_bag

Traceback (most recent call last):
  File "/opt/ros/hydro/lib/python2.7/dist-packages/rqt_bag/bag_widget.py", line 236, in _handle_load_clicked
    self.load_bag(filename[0])
  File "/opt/ros/hydro/lib/python2.7/dist-packages/rqt_bag/bag_widget.py", line 239, in load_bag
    bag = rosbag.Bag(filename)
  File "/opt/ros/hydro/lib/python2.7/dist-packages/rosbag/bag.py", line 159, in __init__
    self._open(f, mode, allow_unindexed)
  File "/opt/ros/hydro/lib/python2.7/dist-packages/rosbag/bag.py", line 890, in _open
    if   mode == 'r': self._open_read(f, allow_unindexed)
  File "/opt/ros/hydro/lib/python2.7/dist-packages/rosbag/bag.py", line 917, in _open_read
    self._reader.start_reading()
  File "/opt/ros/hydro/lib/python2.7/dist-packages/rosbag/bag.py", line 2037, in start_reading
    raise ROSBagUnindexedException()
rosbag.bag.ROSBagUnindexedException: Unindexed bag

Run "rosbag reindex" to index the bagfile.