Manage Alert Groups doesn't allow to update existing group

[#1]

PROBLEM:

Click Alerts and then Actions > Manage Alert Group -> Custom Alert Group.

Then, click the + sign on the right side and pick any alert definition and press OK. Click Save and you will see the 500(server error) Server Error on the alert group screen:

And in the ambari-server.log there is error:

WARN [qtp-ambari-client-510524] ServletHandler:563 - /api/v1/clusters/<cluster-name>/alert_groups/155
java.util.ConcurrentModificationException
 at java.util.HashMap$HashIterator.nextNode(HashMap.java:1429)
 at java.util.HashMap$KeyIterator.next(HashMap.java:1453)
 at org.eclipse.persistence.indirection.IndirectSet$1.next(IndirectSet.java:471)
 at org.apache.ambari.server.orm.entities.AlertGroupEntity.setAlertTargets(AlertGroupEntity.java:313)
 at org.apache.ambari.server.controller.internal.AlertGroupResourceProvider.updateAlertGroups(AlertGroupResourceProvider.java:344)
 at org.apache.ambari.server.controller.internal.AlertGroupResourceProvider.access$100(AlertGroupResourceProvider.java:60)
 at org.apache.ambari.server.controller.internal.AlertGroupResourceProvider$2.invoke(AlertGroupResourceProvider.java:187)
 at org.apache.ambari.server.controller.internal.AlertGroupResourceProvider$2.invoke(AlertGroupResourceProvider.java:184)
 at org.apache.ambari.server.controller.internal.AbstractResourceProvider.invokeWithRetry(AbstractResourceProvider.java:450)
 at org.apache.ambari.server.controller.internal.AbstractResourceProvider.modifyResources(AbstractResourceProvider.java:331)

ROOT CAUSE: https://issues.apache.org/jira/browse/AMBARI-19259

RESOLUTION: Upgrade Ambari to 2.5