perf: Fix a NPE when getting the shell to show a message dialog.

Change-Id: I5159d8460d452c27d265b87c8e4f5db54ba59fad
Signed-off-by: Roberto Oliveira <rdutra@linux.vnet.ibm.com>
Reviewed-on: https://git.eclipse.org/r/38419
Tested-by: Hudson CI
Reviewed-by: Alexander Kurtakov <akurtako@redhat.com>
Tested-by: Alexander Kurtakov <akurtako@redhat.com>