Migration of a Process waiting in a Multi Instance Activity fails if the Process was started before 7.3

Reproduce

• Start a process instance on Camunda 7.2 which waits in a Multi Instance activity
• Upgrade to Camunda 7.5
• Migrate Process Instance

Error

• parallel MI:

  org.camunda.bpm.engine.migration.MigratingProcessInstanceValidationException: ENGINE-23004 Cannot migrate process instance '1775':
          Process instance contains not migrated variables: [1779, 1783, 1781]
  
          at org.camunda.bpm.engine.impl.migration.MigrationLogger.failingMigratingProcessInstanceValidation(MigrationLogger.java:63)
          at org.camunda.bpm.engine.impl.migration.MigrateProcessInstanceCmd.migrateProcessInstance(MigrateProcessInstanceCmd.java:139)
          at org.camunda.bpm.engine.impl.migration.MigrateProcessInstanceCmd$1.call(MigrateProcessInstanceCmd.java:111)
          at org.camunda.bpm.engine.impl.migration.MigrateProcessInstanceCmd$1.call(MigrateProcessInstanceCmd.java:106)
          at org.camunda.bpm.engine.impl.interceptor.CommandContext.runWithoutAuthorization(CommandContext.java:506)
          at org.camunda.bpm.engine.impl.migration.MigrateProcessInstanceCmd.execute(MigrateProcessInstanceCmd.java:106)
          at org.camunda.bpm.engine.impl.migration.MigrateProcessInstanceCmd.execute(MigrateProcessInstanceCmd.java:72)
          at org.camunda.bpm.engine.impl.interceptor.CommandExecutorImpl.execute(CommandExecutorImpl.java:24)
          at org.camunda.bpm.engine.impl.interceptor.CommandContextInterceptor.execute(CommandContextInterceptor.java:104)
          at org.camunda.bpm.engine.impl.interceptor.ProcessApplicationContextInterceptor.execute(ProcessApplicationContextInterceptor.java:66)
          at org.camunda.bpm.engine.impl.interceptor.LogInterceptor.execute(LogInterceptor.java:30)
          at org.camunda.bpm.engine.impl.migration.MigrationPlanExecutionBuilderImpl.execute(MigrationPlanExecutionBuilderImpl.java:97)
          at org.camunda.bpm.engine.impl.migration.MigrationPlanExecutionBuilderImpl.execute(MigrationPlanExecutionBuilderImpl.java:93)
          at org.camunda.bpm.qa.upgrade.scenarios720.multiinstance.MultiInstanceReceiveTaskScenarioTest.testInitParallelMigration(MultiInstanceReceiveTaskScenarioTest.java:83)
  

• sequential MI:

  org.camunda.bpm.engine.exception.NullValueException: The variable "nrOfInstances" could not be found in execution with id 7803: value is null
          at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
          at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
          at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
          at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
          at org.camunda.bpm.engine.impl.util.EnsureUtil.generateException(EnsureUtil.java:334)
          at org.camunda.bpm.engine.impl.util.EnsureUtil.ensureNotNull(EnsureUtil.java:49)
          at org.camunda.bpm.engine.impl.util.EnsureUtil.ensureNotNull(EnsureUtil.java:44)
          at org.camunda.bpm.engine.impl.bpmn.behavior.MultiInstanceActivityBehavior.getLoopVariable(MultiInstanceActivityBehavior.java:194)
          at org.camunda.bpm.engine.impl.bpmn.behavior.SequentialMultiInstanceActivityBehavior.createInnerInstance(SequentialMultiInstanceActivityBehavior.java:92)
          at org.camunda.bpm.engine.impl.migration.instance.MigratingActivityInstance$MigratingScopeActivityInstanceBehavior.createAttachableExecution(MigratingActivityInstance.java:639)
          at org.camunda.bpm.engine.impl.migration.instance.MigratingActivityInstance.createAttachableExecution(MigratingActivityInstance.java:380)
          at org.camunda.bpm.engine.impl.migration.instance.MigratingActivityInstance$MigratingScopeActivityInstanceBehavior.attachState(MigratingActivityInstance.java:538)
          at org.camunda.bpm.engine.impl.migration.instance.MigratingActivityInstance.attachState(MigratingActivityInstance.java:165)
          at org.camunda.bpm.engine.impl.migration.instance.MigratingProcessElementInstanceVisitor.migrateProcessElementInstance(MigratingProcessElementInstanceVisitor.java:80)
          at org.camunda.bpm.engine.impl.migration.instance.MigratingProcessElementInstanceVisitor.visit(MigratingProcessElementInstanceVisitor.java:33)
          at org.camunda.bpm.engine.impl.migration.instance.MigratingProcessElementInstanceVisitor.visit(MigratingProcessElementInstanceVisitor.java:28)
          at org.camunda.bpm.engine.impl.tree.ReferenceWalker.walkUntil(ReferenceWalker.java:87)
          at org.camunda.bpm.engine.impl.tree.ReferenceWalker.walkUntil(ReferenceWalker.java:65)
          at org.camunda.bpm.engine.impl.migration.MigrateProcessInstanceCmd.migrateProcessInstance(MigrateProcessInstanceCmd.java:325)
          at org.camunda.bpm.engine.impl.migration.MigrateProcessInstanceCmd$3.run(MigrateProcessInstanceCmd.java:155)
          at org.camunda.bpm.engine.impl.migration.MigrateProcessInstanceCmd.executeInContext(MigrateProcessInstanceCmd.java:176)
          at org.camunda.bpm.engine.impl.migration.MigrateProcessInstanceCmd.migrateProcessInstance(MigrateProcessInstanceCmd.java:151)
          at org.camunda.bpm.engine.impl.migration.MigrateProcessInstanceCmd$1.call(MigrateProcessInstanceCmd.java:111)
          at org.camunda.bpm.engine.impl.migration.MigrateProcessInstanceCmd$1.call(MigrateProcessInstanceCmd.java:106)
          at org.camunda.bpm.engine.impl.interceptor.CommandContext.runWithoutAuthorization(CommandContext.java:506)
          at org.camunda.bpm.engine.impl.migration.MigrateProcessInstanceCmd.execute(MigrateProcessInstanceCmd.java:106)
          at org.camunda.bpm.engine.impl.migration.MigrateProcessInstanceCmd.execute(MigrateProcessInstanceCmd.java:72)
          at org.camunda.bpm.engine.impl.interceptor.CommandExecutorImpl.execute(CommandExecutorImpl.java:24)
          at org.camunda.bpm.engine.impl.interceptor.CommandContextInterceptor.execute(CommandContextInterceptor.java:104)
          at org.camunda.bpm.engine.impl.interceptor.ProcessApplicationContextInterceptor.execute(ProcessApplicationContextInterceptor.java:66)
          at org.camunda.bpm.engine.impl.interceptor.LogInterceptor.execute(LogInterceptor.java:30)
          at org.camunda.bpm.engine.impl.migration.MigrationPlanExecutionBuilderImpl.execute(MigrationPlanExecutionBuilderImpl.java:97)
          at org.camunda.bpm.engine.impl.migration.MigrationPlanExecutionBuilderImpl.execute(MigrationPlanExecutionBuilderImpl.java:93)
          at org.camunda.bpm.qa.upgrade.scenarios720.multiinstance.MultiInstanceReceiveTaskScenarioTest.testInitSequentialMigration(MultiInstanceReceiveTaskScenarioTest.java:145)
  

AT

• I can migrate MI actvities from before 7.3 to after 7.5

Hint

• with 7.3 the MI implementation was refactored and added an additional scope:
  https://blog.camunda.org/post/2015/06/why-we-re-implemented-bpmn-multi/
• failing test cases: https://github.com/camunda/camunda-bpm-platform/commit/5bd75b31ce8cf73deaeb7959b6025cb8acd16491