Issue #2468: [ObjectDB 2.8.1] Unexpected exception (Error 990)

[ObjectDB 2.8.1] Unexpected exception (Error 990)
Generated by OpenJDK 64-Bit Server VM 1.8.0_212 (on Linux 5.0.0-27-generic).
Please report this error on http://www.objectdb.com/issue/new
com.objectdb.o.InternalException: java.lang.NullPointerException: null java.lang.NullPointerException
at com.objectdb.o.OBM.aO(OBM.java:396)
at com.objectdb.o.OBM.aO(OBM.java:270)
at com.objectdb.jpa.EMImpl.persist(EMImpl.java:415)
at javax.persistence.EntityManager$persist$1.call(Unknown Source)
at org.codehaus.groovy.runtime.callsite.CallSiteArray.defaultCall(CallSiteArray.java:47)
at org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:115)
at org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:127)
at common.app.Model.save(Model.groovy:17) at common.app.Model.saveAndFlush(Model.groovy:21)
at common.app.Model$saveAndFlush.call(Unknown Source)
at org.codehaus.groovy.runtime.callsite.CallSiteArray.defaultCall(CallSiteArray.java:47)
at org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:115)
at org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:119)
at common.app.AuQuery.getSite(AuQuery.groovy:111)
at common.app.AuQuery$getSite.call(Unknown Source)
at org.codehaus.groovy.runtime.callsite.CallSiteArray.defaultCall(CallSiteArray.java:47)
at org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:115)
at org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:119)
at common.app.AuMain$_start_closure1.doCall(AuMain.groovy:28)
at common.app.AuMain$_start_closure1.doCall(AuMain.groovy)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.codehaus.groovy.reflection.CachedMethod.invoke(CachedMethod.java:123)
at groovy.lang.MetaMethod.doMethodInvoke(MetaMethod.java:323)
at org.codehaus.groovy.runtime.metaclass.ClosureMetaClass.invokeMethod(ClosureMetaClass.java:263)
at groovy.lang.MetaClassImpl.invokeMethod(MetaClassImpl.java:1048)
at org.codehaus.groovy.runtime.callsite.PogoMetaClassSite.call(PogoMetaClassSite.java:37)
at org.codehaus.groovy.runtime.callsite.CallSiteArray.defaultCall(CallSiteArray.java:47)
at org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:115)
at org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:119)
at common.app.Database.transact(Database.groovy:77)
at common.app.Database$transact$0.call(Unknown Source)
at org.codehaus.groovy.runtime.callsite.CallSiteArray.defaultCall(CallSiteArray.java:47)
at org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:115)
at org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:127)
at common.app.AuMain.start(AuMain.groovy:27) at common.app.AuMain.main(AuMain.groovy:57)

Thank you for this report, which seems to be a duplicate of issue #2449.