process does not end on exit

[Moses Moore]

Steps to reproduce:
`/usr/games/einstein`
Click on 'exit'.

Expected outcome:
Program ends, process removed.

Witnessed outcome:
This is written to stdout:

    X Error of failed request: BadWindow (invalid Window parameter)
    Major opcode of failed request: 4 (X_DestroyWindow)
    Resource id in failed request: 0x1a0000f
    Serial number of failed request: 224
    Current serial number in output stream: 228

Process does not end until I do `kill $pidof einstein)` or ctrl-c on stdin.

Other info:
high (~100%) CPU use even when the main window is gone. strace says the last syscalls are:

    poll([{fd=3, events=POLLIN|POLLOUT}], 1, -1) = 1 ([{fd=3, revents=POLLOUT}])
    writev(3, [{"\2\16\4\0\17\0\240\1\0@\0\0\t\0\240\1+$\1\0", 20}, {NULL, 0}, {"", 0}], 3) = 20

file descriptor #3 is, I think, unix socket "/tmp/.X11-unix/X0"

ProblemType: Bug
DistroRelease: Ubuntu 15.04
Package: einstein 2.0.dfsg.2-9
ProcVersionSignature: Ubuntu 3.19.0-21.21-generic 3.19.8
Uname: Linux 3.19.0-21-generic i686
NonfreeKernelModules: nvidia
ApportVersion: 2.17.2-0ubuntu1.1
Architecture: i386
CurrentDesktop: XFCE
Date: Sun Jul 12 18:48:17 2015
InstallationDate: Installed on 2013-06-08 (764 days ago)
InstallationMedia: Xubuntu 13.04 "Raring Ringtail" - Release i386 (20130423.1)
SourcePackage: einstein
UpgradeStatus: Upgraded to vivid on 2015-04-25 (78 days ago)