Live Merge fails at the very last stage with a Null Pointer Exception

Issue

• Live Merge (snapshot deletion) failed.

• The engine logs contained a Null Pointer Exception (NPE);

2019-02-22 03:36:06,659+01 ERROR [org.ovirt.engine.core.bll.tasks.CommandAsyncTask] (org.ovirt.thread.pool-6-thread-29) [82087b87-dd81-4eb7-afd5-7ba518521148] [within thread]: endAction for action type DestroyImage threw an exception.: org.apache.commons.lang.SerializationException: org.codehaus.jackson.map.JsonMappingException: (was java.lang.NullPointerException) (through reference chain: org.ovirt.engine.core.common.action.DestroyImageParameters["liveMerge"])
        at org.ovirt.engine.core.utils.serialization.json.JsonObjectSerializer.writeJsonAsString(JsonObjectSerializer.java:80) [utils.jar:]
        at org.ovirt.engine.core.utils.serialization.json.JsonObjectSerializer.serialize(JsonObjectSerializer.java:63) [utils.jar:]
        ............

• The merge actually completed successfully on the host.

• The original 'top' volume of the merge had been removed and the volume metadata of the parent volume had been modified correctly.