Fix balance-masters functionality in the new LeadershipService based device mastership store

Change-Id: I9f64d514cee7d5a5383fd4c2fa30a8616c97785c
diff --git a/core/net/src/main/java/org/onosproject/net/device/impl/DeviceManager.java b/core/net/src/main/java/org/onosproject/net/device/impl/DeviceManager.java
index 58d0512..d975d8a 100644
--- a/core/net/src/main/java/org/onosproject/net/device/impl/DeviceManager.java
+++ b/core/net/src/main/java/org/onosproject/net/device/impl/DeviceManager.java
@@ -536,7 +536,7 @@
         if (myNextRole == NONE) {
             mastershipService.requestRoleFor(did);
             MastershipTerm term = termService.getMastershipTerm(did);
-            if (myNodeId.equals(term.master())) {
+            if (term != null && myNodeId.equals(term.master())) {
                 myNextRole = MASTER;
             } else {
                 myNextRole = STANDBY;