Bug 443092 -[Contributions] NPE in
 DirectContributionItem.getExecutionContext(IEclipseContext context)

 Added guard for null in getExecutionContext.

 This bug was previously concealed by bug 442570, once that bug is fixed
 this bug manifests every time a menu with dynamic items is shown

Change-Id: Ib9fdbc3c0dfb215b193acebd2e592a468f0fbfdd
Signed-off-by: Bruce Skingle <Bruce.Skingle@immutify.com>