Shutdown/Restart prompt may stop working

Impact
======
The Log Out/Power Off (software) buttons stop working in Ubuntu 16.04 LTS.

Test Case
=========
1. From Ubuntu GNOME, open the gedit Text Editor.
2. Type a few characters but do not save the file.
3. In the top right corner, click the system status menu. Click your name and then Log Out.
4. Click the gedit icon in the Log Out dialog, then click Cancel in the Log Out dialog.
5. Repeat step 3.

If the bug happens, the Log Out dialog does not appear after Step 5.

I was not able to trigger this bug every time but I think this test case makes it more likely to trigger the bug

Original Bug Report
===================
The shutdown/restart prompt(60 seconds or the system will shutdown automatically) may stop working. Can be triggered by simply clicking on the button to open the prompt and not selecting an option or by opening the prompt multiple times.

Description: Ubuntu 16.04 LTS
Release: 16.04

gnome-shell:
  Installed: 3.18.4-0ubuntu3
  Candidate: 3.18.4-0ubuntu3
  Version table:
 *** 3.18.4-0ubuntu3 500
        500 http://us.archive.ubuntu.com/ubuntu xenial/universe amd64 Packages
        100 /var/lib/dpkg/status

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: gnome-shell 3.18.4-0ubuntu3
ProcVersionSignature: Ubuntu 4.4.0-21.37-generic 4.4.6
Uname: Linux 4.4.0-21-generic x86_64
NonfreeKernelModules: nvidia_uvm nvidia_drm nvidia_modeset nvidia
ApportVersion: 2.20.1-0ubuntu2
Architecture: amd64
CurrentDesktop: GNOME
Date: Tue Apr 26 19:18:31 2016
DisplayManager: gdm3
InstallationDate: Installed on 2016-04-23 (3 days ago)
InstallationMedia: Ubuntu-GNOME 16.04 LTS "Xenial Xerus" - Release amd64 (20160421)
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: gnome-shell
UpgradeStatus: No upgrade log present (probably fresh install)