Fixed null providers & custom topo simulator to work across stop/starts.

Change-Id: Id8eeca73c8fc021c351d365ef009b3a95562763a
diff --git a/providers/null/src/main/java/org/onosproject/provider/nil/NullProviders.java b/providers/null/src/main/java/org/onosproject/provider/nil/NullProviders.java
index 7313f12..83471a5 100644
--- a/providers/null/src/main/java/org/onosproject/provider/nil/NullProviders.java
+++ b/providers/null/src/main/java/org/onosproject/provider/nil/NullProviders.java
@@ -385,6 +385,7 @@
             delay(500);
             simulator.tearDownTopology();
             simulator = null;
+
         }
     }