Can you share the stacktrace?
Right after where it says the NullPointerException, it should say where this occurred. You can look up that java action and go to that specific line to hopefully see what was 'Null' there.'
Edit:
Here's an example.
In this case, the error would be in the DomainModelUtil.scala action on line 69. (see highlight).
If you look this up in your stacktrace, and go to that action+line, you might be able to find what object is null.
ERROR - Connector: (14/81) Caused by: com.mendix.core.CoreRuntimeException: 'TestEntity.Entity' is not a persistable entity ERROR - Connector: (15/81) #011at com.mendix.connectionbus.DomainModelUtilImpl.getPersistableMetaObject(DomainModelUtil.scala:69) ERROR - Connector: (16/81) #011at com.mendix.connectionbus.DomainModelUtil$.getPersistableMetaObject(DomainModelUtil.scala:31) ERROR - Connector: (17/81) #011at com.mendix.connectionbus.DomainModelUtil.getPersistableMetaObject(DomainModelUtil.scala) ERROR - Connector: (18/81) #011at com.mendix.connectionbus.retrieve.query.mapping.MetaObjectInfo.getMetaObject(MetaObjectInfo.java:138) ERROR - Connector: (19/81) #011at com.mendix.connectionbus.retrieve.query.mapping.MetaObjectInfo.getLocation(MetaObjectInfo.java:102) ERROR - Connector: (20/81) #011at com.mendix.connectionbus.retrieve.query.mapping.MetaObjectInfo.getIdLocation(MetaObjectInfo.java:86) ERROR - Connector: (21/81) #011at com.mendix.connectionbus.retrieve.query.mapping.MetaObjectInfo.getIdLocation(MetaObjectInfo.java:82) ERROR - Connector: (22/81) #011at com.mendix.connectionbus.retrieve.query.mapping.MetaObjectInfo.getIdLocation(MetaObjectInfo.java:78)