Fixed NPE in sending push notifications

components/device-mgt/org.wso2.carbon.device.mgt.core/src/main/java/org/wso2/carbon/device/mgt/core/push/notification/mgt/task/PushNotificationSchedulerTask.java

@@ -86,11 +86,18 @@ public class PushNotificationSchedulerTask implements Runnable {
                         NotificationStrategy notificationStrategy = provider.getNotificationStrategyByDeviceType
                                 (operationMapping.getDeviceIdentifier().getType());
                         // Send the push notification on given strategy
+                        if (notificationStrategy != null) {
                             notificationStrategy.execute(new NotificationContext(operationMapping.getDeviceIdentifier(),
                                     provider.getOperation(operationMapping.getDeviceIdentifier().getType(), operationMapping
                                             .getOperationId())));
                             operationMapping.setPushNotificationStatus(Operation.PushNotificationStatus.COMPLETED);
                             operationsCompletedList.add(operationMapping);
+                        } else {
+                            if (log.isDebugEnabled()) {
+                                log.debug("Tenant '" + PrivilegedCarbonContext.getThreadLocalCarbonContext()
+                                        .getTenantDomain() + "' does not have push notification strategy.");
+                            }
+                        }
                     } catch (DeviceManagementException e) {
                         log.error("Error occurred while getting notification strategy for operation mapping " +
                                 operationMapping.getDeviceIdentifier().getType(), e);
@@ -121,7 +128,7 @@ public class PushNotificationSchedulerTask implements Runnable {
                 log.debug("Push notification job running completed.");
             }
         } catch (Throwable cause) {
-            log.error("PushNotificationSchedulerTask failed due to " + cause);
+            log.error("PushNotificationSchedulerTask failed due to " + cause.getMessage(), cause);
         }
     }
 }