Changes

[510279] Fix potential NPE in AbstractEEFWidgetLifecycleManager

AbstractEEFWidgetLifecycleManager.lockedBy* are called asynchronously
and sometimes this can occur at a time when the ControlDecoration has
been disposed, causing NPEs in ControlDecoration.setImage() when it
accesses control.getShell(). ControlDecoration does not have an
isDisposed() method, but testing for getControl() != null is
equivalent.

Bug: https://bugs.eclipse.org/bugs/show_bug.cgi?id=510279
Change-Id: Icf155151eacbdbf50db4161955222f566dadf70e
Signed-off-by: Stéphane Bégaudeau <stephane.begaudeau@obeo.fr>