Use canonical names of services insread of strings.

src/main/java/net/floodlightcontroller/core/web/EventHistoryTopologyClusterResource.java

 package net.floodlightcontroller.core.web;
 
+import net.floodlightcontroller.linkdiscovery.ILinkDiscoveryService;
 import net.floodlightcontroller.linkdiscovery.internal.EventHistoryTopologyCluster;
 import net.floodlightcontroller.linkdiscovery.internal.LinkDiscoveryManager;
 import net.floodlightcontroller.util.EventHistory;
@@ -27,7 +28,8 @@ public class EventHistoryTopologyClusterResource extends ServerResource {
         }
 
         LinkDiscoveryManager topoManager =
-           (LinkDiscoveryManager)getContext().getAttributes().get("topology");
+           (LinkDiscoveryManager)getContext().getAttributes().
+               get(ILinkDiscoveryService.class.getCanonicalName());
 
         return new EventHistory<EventHistoryTopologyCluster>(
                                 topoManager.evHistTopologyCluster, count);

src/main/java/net/floodlightcontroller/core/web/EventHistoryTopologyLinkResource.java

 package net.floodlightcontroller.core.web;
 
+import net.floodlightcontroller.linkdiscovery.ILinkDiscoveryService;
 import net.floodlightcontroller.linkdiscovery.internal.EventHistoryTopologyLink;
 import net.floodlightcontroller.linkdiscovery.internal.LinkDiscoveryManager;
 import net.floodlightcontroller.util.EventHistory;
@@ -27,7 +28,8 @@ public class EventHistoryTopologyLinkResource extends ServerResource {
         }
 
         LinkDiscoveryManager topoManager =
-           (LinkDiscoveryManager)getContext().getAttributes().get("topology");
+           (LinkDiscoveryManager)getContext().getAttributes().
+               get(ILinkDiscoveryService.class.getCanonicalName());
 
         return new EventHistory<EventHistoryTopologyLink>(
                                     topoManager.evHistTopologyLink, count);

src/main/java/net/floodlightcontroller/core/web/EventHistoryTopologySwitchResource.java

 package net.floodlightcontroller.core.web;
 
+import net.floodlightcontroller.linkdiscovery.ILinkDiscoveryService;
 import net.floodlightcontroller.linkdiscovery.internal.EventHistoryTopologySwitch;
 import net.floodlightcontroller.linkdiscovery.internal.LinkDiscoveryManager;
 import net.floodlightcontroller.util.EventHistory;
@@ -27,7 +28,8 @@ public class EventHistoryTopologySwitchResource extends ServerResource {
         }
 
         LinkDiscoveryManager topoManager =
-           (LinkDiscoveryManager)getContext().getAttributes().get("topology");
+           (LinkDiscoveryManager)getContext().getAttributes().
+               get(ILinkDiscoveryService.class.getCanonicalName());
 
         return new EventHistory<EventHistoryTopologySwitch>(
                                 topoManager.evHistTopologySwitch, count);

src/main/java/net/floodlightcontroller/util/EventHistory.java

@@ -90,6 +90,11 @@ public class EventHistory<T> {
 
     // Copy constructor - copy latest k items of the event history
     public EventHistory(EventHistory<T> eventHist, int latestK) {
+
+        if (eventHist == null) {
+            description = "No event found";
+            return;
+        }
         int curSize = (eventHist.full)?eventHist.event_history_size:
                                                     eventHist.current_index;
         int size  = (latestK < curSize)?latestK:curSize;