fix for mastership race condition.

Change-Id: Id219bdf00f3d6501027566d54342c5d7c1b27789

@@ -614,7 +614,11 @@ public class DeviceManager
             }
 
             // device is connected to this node:
-            reassertRole(did, myNextRole);
+            if (store.getDevice(did) != null) {
+                reassertRole(did, myNextRole);
+            } else {
+                log.warn("Device is not yet/no longer in the store: {}", did);
+            }
         }
     }