NullPointerException when creating a group membership for non-existing user

Component

https://github.com/camunda/camunda-bpm-platform/tree/master/engine

Steps to reproduce

• Create a group membership via identity service (IdentityService#createMembership for a user that does not exist.

Observed Behavior

• NullPointerException with the following stacktrace:

  09:54:07.143 [main] ERROR org.camunda.bpm.engine.context - ENGINE-16004 Exception while closing command context: ENGINE-03083 Unexpected exception while executing database operations with message '
  ### Error updating database.  Cause: java.lang.NullPointerException
  ### The error may involve org.camunda.bpm.engine.impl.persistence.entity.MembershipRelation.insertMembership-Inline
  ### The error occurred while setting parameters
  ### SQL: insert into ACT_ID_MEMBERSHIP (USER_ID_, GROUP_ID_)     values (       ?,       ?     )
  ### Cause: java.lang.NullPointerException'. Flush summary: 
   [
    INSERT MembershipEntity[7]
  ]
  org.camunda.bpm.engine.ProcessEngineException: ENGINE-03083 Unexpected exception while executing database operations with message '
  ### Error updating database.  Cause: java.lang.NullPointerException
  ### The error may involve org.camunda.bpm.engine.impl.persistence.entity.MembershipRelation.insertMembership-Inline
  ### The error occurred while setting parameters
  ### SQL: insert into ACT_ID_MEMBERSHIP (USER_ID_, GROUP_ID_)     values (       ?,       ?     )
  ### Cause: java.lang.NullPointerException'. Flush summary: 
   [
    INSERT MembershipEntity[7]
  ]
  	at org.camunda.bpm.engine.impl.db.EnginePersistenceLogger.flushDbOperationsException(EnginePersistenceLogger.java:693) ~[classes/:na]
  	at org.camunda.bpm.engine.impl.db.entitymanager.DbEntityManager.flushDbOperations(DbEntityManager.java:341) ~[classes/:na]
  	at org.camunda.bpm.engine.impl.db.entitymanager.DbEntityManager.flushDbOperationManager(DbEntityManager.java:322) ~[classes/:na]
  	at org.camunda.bpm.engine.impl.db.entitymanager.DbEntityManager.flush(DbEntityManager.java:294) ~[classes/:na]
  	at org.camunda.bpm.engine.impl.interceptor.CommandContext.flushSessions(CommandContext.java:263) ~[classes/:na]
  	at org.camunda.bpm.engine.impl.interceptor.CommandContext.close(CommandContext.java:186) ~[classes/:na]
  	at org.camunda.bpm.engine.impl.interceptor.CommandContextInterceptor.execute(CommandContextInterceptor.java:119) [classes/:na]
  	at org.camunda.bpm.engine.impl.interceptor.ProcessApplicationContextInterceptor.execute(ProcessApplicationContextInterceptor.java:70) [classes/:na]
  	at org.camunda.bpm.engine.impl.interceptor.LogInterceptor.execute(LogInterceptor.java:33) [classes/:na]
  	at org.camunda.bpm.engine.impl.IdentityServiceImpl.createMembership(IdentityServiceImpl.java:158) [classes/:na]
  	at org.camunda.bpm.engine.test.api.task.TaskQueryOrTest.shouldReturnTasksWithTaskCandidateUserOrTaskCandidateGroup(TaskQueryOrTest.java:231) [test-classes/:na]
  	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_172]
  	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_172]
  	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_172]
  	at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_172]
  	at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50) [junit-4.12.jar:4.12]
  	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12) [junit-4.12.jar:4.12]
  	at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47) [junit-4.12.jar:4.12]
  	at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17) [junit-4.12.jar:4.12]
  	at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26) [junit-4.12.jar:4.12]
  	at org.junit.internal.runners.statements.RunAfters.evaluate(RunAfters.java:27) [junit-4.12.jar:4.12]
  	at org.junit.rules.TestWatcher$1.evaluate(TestWatcher.java:55) [junit-4.12.jar:4.12]
  	at org.camunda.bpm.engine.test.ProcessEngineRule$1.evaluate(ProcessEngineRule.java:172) [classes/:na]
  	at org.junit.rules.ExpectedException$ExpectedExceptionStatement.evaluate(ExpectedException.java:239) [junit-4.12.jar:4.12]
  	at org.junit.rules.RunRules.evaluate(RunRules.java:20) [junit-4.12.jar:4.12]
  	at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325) [junit-4.12.jar:4.12]
  	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78) [junit-4.12.jar:4.12]
  	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57) [junit-4.12.jar:4.12]
  	at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290) [junit-4.12.jar:4.12]
  	at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71) [junit-4.12.jar:4.12]
  	at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288) [junit-4.12.jar:4.12]
  	at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58) [junit-4.12.jar:4.12]
  	at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268) [junit-4.12.jar:4.12]
  	at org.junit.runners.ParentRunner.run(ParentRunner.java:363) [junit-4.12.jar:4.12]
  	at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:86) [.cp/:na]
  	at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38) [.cp/:na]
  	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:538) [.cp/:na]
  	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:760) [.cp/:na]
  	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:460) [.cp/:na]
  	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:206) [.cp/:na]
  Caused by: org.apache.ibatis.exceptions.PersistenceException: 
  ### Error updating database.  Cause: java.lang.NullPointerException
  ### The error may involve org.camunda.bpm.engine.impl.persistence.entity.MembershipRelation.insertMembership-Inline
  ### The error occurred while setting parameters
  ### SQL: insert into ACT_ID_MEMBERSHIP (USER_ID_, GROUP_ID_)     values (       ?,       ?     )
  ### Cause: java.lang.NullPointerException
  	at org.apache.ibatis.exceptions.ExceptionFactory.wrapException(ExceptionFactory.java:30) ~[mybatis-3.4.4.jar:3.4.4]
  	at org.apache.ibatis.session.defaults.DefaultSqlSession.update(DefaultSqlSession.java:200) ~[mybatis-3.4.4.jar:3.4.4]
  	at org.apache.ibatis.session.defaults.DefaultSqlSession.insert(DefaultSqlSession.java:185) ~[mybatis-3.4.4.jar:3.4.4]
  	at org.camunda.bpm.engine.impl.db.sql.DbSqlSession.executeInsertEntity(DbSqlSession.java:257) ~[classes/:na]
  	at org.camunda.bpm.engine.impl.db.sql.DbSqlSession.insertEntity(DbSqlSession.java:252) ~[classes/:na]
  	at org.camunda.bpm.engine.impl.db.AbstractPersistenceSession.executeDbOperation(AbstractPersistenceSession.java:44) ~[classes/:na]
  	at org.camunda.bpm.engine.impl.db.sql.BatchDbSqlSession.executeDbOperations(BatchDbSqlSession.java:61) ~[classes/:na]
  	at org.camunda.bpm.engine.impl.db.entitymanager.DbEntityManager.flushDbOperations(DbEntityManager.java:339) ~[classes/:na]
  	... 38 common frames omitted
  Caused by: java.lang.NullPointerException: null
  	at org.camunda.bpm.engine.impl.persistence.entity.MembershipEntity.getUserId(MembershipEntity.java:68) ~[classes/:na]
  	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_172]
  	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_172]
  	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_172]
  	at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_172]
  	at org.apache.ibatis.reflection.invoker.MethodInvoker.invoke(MethodInvoker.java:41) ~[mybatis-3.4.4.jar:3.4.4]
  	at org.apache.ibatis.reflection.wrapper.BeanWrapper.getBeanProperty(BeanWrapper.java:164) ~[mybatis-3.4.4.jar:3.4.4]
  	at org.apache.ibatis.reflection.wrapper.BeanWrapper.get(BeanWrapper.java:49) ~[mybatis-3.4.4.jar:3.4.4]
  	at org.apache.ibatis.reflection.MetaObject.getValue(MetaObject.java:122) ~[mybatis-3.4.4.jar:3.4.4]
  	at org.apache.ibatis.scripting.defaults.DefaultParameterHandler.setParameters(DefaultParameterHandler.java:79) ~[mybatis-3.4.4.jar:3.4.4]
  	at org.apache.ibatis.executor.statement.PreparedStatementHandler.parameterize(PreparedStatementHandler.java:93) ~[mybatis-3.4.4.jar:3.4.4]
  	at org.apache.ibatis.executor.statement.RoutingStatementHandler.parameterize(RoutingStatementHandler.java:64) ~[mybatis-3.4.4.jar:3.4.4]
  	at org.apache.ibatis.executor.BatchExecutor.doUpdate(BatchExecutor.java:71) ~[mybatis-3.4.4.jar:3.4.4]
  	at org.apache.ibatis.executor.BaseExecutor.update(BaseExecutor.java:117) ~[mybatis-3.4.4.jar:3.4.4]
  	at org.apache.ibatis.executor.CachingExecutor.update(CachingExecutor.java:76) ~[mybatis-3.4.4.jar:3.4.4]
  	at org.apache.ibatis.session.defaults.DefaultSqlSession.update(DefaultSqlSession.java:198) ~[mybatis-3.4.4.jar:3.4.4]
  	... 44 common frames omitted
  

Expected behavior

• A proper ProcessEngineException is thrown that explains in the message that the user does not exist

Implementation notes

• Check if the same is true for groups that do not exist. And fix it in that case as well.