Fix NPE on shutdown if providers are already gone

Change-Id: I019786f26a589a5b82b72f736fa2d148571ea274

@@ -74,7 +74,6 @@ import com.google.common.collect.Maps;
 import com.google.common.collect.Sets;
 import com.google.common.util.concurrent.Futures;
 
-import static com.google.common.base.Preconditions.checkNotNull;
 import static org.onosproject.net.DefaultAnnotations.merge;
 import static org.onosproject.net.DefaultAnnotations.union;
 import static org.onosproject.net.Link.State.ACTIVE;
@@ -353,7 +352,12 @@ public class ECLinkStore
 
     private Link composeLink(LinkKey linkKey) {
 
-        ProviderId baseProviderId = checkNotNull(getBaseProviderId(linkKey));
+        ProviderId baseProviderId = getBaseProviderId(linkKey);
+        if (baseProviderId == null) {
+            // provider was not found, this means it was already removed by the
+            // parent component.
+            return null;
+        }
         LinkDescription base = linkDescriptions.get(new Provided<>(linkKey, baseProviderId));
         // short circuit if link description no longer exists
         if (base == null) {