Group ID was set to null in a DefaultFlowRule constructor.

This is assumed to be not null in some places.

Change-Id: Id3c7f2e71b658c0077f495bf61ed997c98e16acd
diff --git a/core/api/src/main/java/org/onosproject/net/flow/DefaultFlowRule.java b/core/api/src/main/java/org/onosproject/net/flow/DefaultFlowRule.java
index 1c13ef0..9bc192b 100644
--- a/core/api/src/main/java/org/onosproject/net/flow/DefaultFlowRule.java
+++ b/core/api/src/main/java/org/onosproject/net/flow/DefaultFlowRule.java
@@ -15,17 +15,17 @@
  */
 package org.onosproject.net.flow;
 
-import static com.google.common.base.MoreObjects.toStringHelper;
-import static com.google.common.base.Preconditions.checkArgument;
-import static com.google.common.base.Preconditions.checkNotNull;
-
-import java.util.Objects;
-
 import org.onosproject.core.ApplicationId;
 import org.onosproject.core.DefaultGroupId;
 import org.onosproject.core.GroupId;
 import org.onosproject.net.DeviceId;
 
+import java.util.Objects;
+
+import static com.google.common.base.MoreObjects.toStringHelper;
+import static com.google.common.base.Preconditions.checkArgument;
+import static com.google.common.base.Preconditions.checkNotNull;
+
 public class DefaultFlowRule implements FlowRule {
 
     private final DeviceId deviceId;
@@ -191,7 +191,7 @@
 
 
         //FIXME: fields below will be removed.
-        this.groupId = null;
+        this.groupId = new DefaultGroupId(0);
         this.payLoad = null;
     }