Oracle Enterprise Manager : Internal error detected: oracle.sysman.gcagent.task.TaskZombieException

I was getting a lot of error messages/incidents and alerts stating 'Internal error detected: oracle.sysman.gcagent.task.TaskZombieException'
in my Enterprise Manager (Release 13cR2)

The right way should be to find out what is causing these (generally failed / timed out responses)
However, they are also caused due to bugs in OEM.

The way to suppress these is as below 

# cd $AGENT_HOME/agent_inst

# bin/emctl stop agent
# cd sysman/config
# cp emd.properties emd.properties.bkp_19Aug2019
# vi emd.properties

Append Below 

_zombieSuspensions=true
_canceledThreadWait=900
_zombieThreadPercentThreshold=0
_zombieCreateIncident=false
_cancelThreadTimeout=300
_traceSeverityForTypes=noTypes

# cp s_jvm_options.opt s_jvm_options.opt.bkp_19Aug2019
# vi s_jvm_options.opt

Append Below at EOF

-XX:+UseMembar


# $AGENT_HOME/agent_inst/bin/emctl start agent

this will stop creating incidents for your zombie processes. 
Reference - MoS - Doc ID 1468586.1