When the workbench is shut down, e.g. due to an unsuccessful start, then
the active workbench window's shell is already set to null. It is safer
to return the Workbench's Display.

Change-Id: I6d5b1a951b557faff0aff50bf19dc7482fbccee9
Signed-off-by: Karsten Thoms <karsten.thoms@karakun.com>