Workflow is not getting triggered on a custom entity

Issue

• The below errors are observed on the Liferay console whenever trying to apply a 'Single Approval Workflow' on a 'Custom Entity'

ERROR [com.liferay.portal.workflow.metrics.internal.petra.executor.WorkflowMetricsPortalExecutor-2][WorkflowMetricsPortalExecutor:56] java.lang.NullPointerException
java.lang.NullPointerException
at com.liferay.portal.workflow.metrics.internal.search.index.InstanceWorkflowMetricsIndexer._getAssetEntry(InstanceWorkflowMetricsIndexer.java:199)
at com.liferay.portal.workflow.metrics.internal.search.index.InstanceWorkflowMetricsIndexer.createDocument(InstanceWorkflowMetricsIndexer.java:64)
at com.liferay.portal.workflow.metrics.internal.model.listener.KaleoInstanceModelListener.lambda$onAfterUpdate$2(KaleoInstanceModelListener.java:51)
at com.liferay.portal.workflow.metrics.internal.petra.executor.WorkflowMetricsPortalExecutor.lambda$null$0(WorkflowMetricsPortalExecutor.java:53)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)

Environment

• Liferay 7.2 GA1

Resolution

• The issue is resolved in Liferay 7.2 SP1+
• Need to upgrade the portal from Liferay 7.2 GA1 to Liferay DXP 7.2 Fix Pack 2+ (ie: SP1).

Additional Information

• When migrating from 7.2 GA1 to Liferay DXP 7.2 Fix Pack 2+ (ie: SP1) or above, it is required to run the upgrade tool after installing the patch. 
• Updating to Liferay DXP 7.2 SP1 / DXP-2+ from Liferay DXP 7.2 GA1 or Fix Pack 1