When the perspective's parent is null, then the perspective is not part
of the model.
In this case return early.

Change-Id: Ia8828d7a3f5108de20c7e3b3c5eff7fa12cf15a1
Signed-off-by: Karsten Thoms <karsten.thoms@karakun.com>
Reviewed-on: https://git.eclipse.org/r/c/platform/eclipse.platform.ui/+/173911
Tested-by: Platform Bot <platform-bot@eclipse.org>