Add a guard when Display#getActiveShell() returns null. This might
happen in rare situations, e.g. when an e4 view is dragged to detach it.

Change-Id: I1566047b9b65b47039ea80fade5a77d950aaa82f
Signed-off-by: Karsten Thoms <karsten.thoms@karakun.com>
Reviewed-on: https://git.eclipse.org/r/c/platform/eclipse.platform.ui/+/173029
Tested-by: Platform Bot <platform-bot@eclipse.org>
Reviewed-by: Alexander Kurtakov <akurtako@redhat.com>