Following an upgrade from System Center Virtual Machine Manager 2008 R2 to System Center Virtual Machine Manager 2012, the System Center Virtual Machine Manager (SCVMMService) service kept crashing a couple of times an hour.

In the Event Log under Applications and Services Logs there’s a VM Manager log.  In this log two events were logged:

System.NullReferenceException: Object reference not set to an instance of an object.
at Microsoft.VirtualManager.Engine.BitBos.VMRefresherBase.UpdateDVDDrives(IVMComputerSystem vmComputer)
at Microsoft.VirtualManager.Engine.BitBos.VMRefresherBase.UpdateFullVMObjectToCarmine(IVMComputerSystem vmComputer, VMData vmData, UpdateRequired updateRequired, Boolean vmObjectHasChanged)
at Microsoft.VirtualManager.Engine.BitBos.VMRefresherBase.RunOnDemandVmRefresh(Guid vmObjectId, VM tempVM)
at Microsoft.VirtualManager.Engine.BitBos.VMRefresherBase.UpdateHostAndVMs(VMRefresherType refresherType, Guid vmObjectId, VM tempVm)
at Microsoft.VirtualManager.Engine.VmOperations.RefreshVmSubtask.RunSubtask()
at Microsoft.VirtualManager.Engine.TaskRepository.SubtaskBase.Run()
at Microsoft.VirtualManager.Engine.TaskRepository.Task`1.SubtaskRun(Object state)-2147467261

Virtual Machine Manager (vmmservice:1476) has encountered an error and needed to exit the process. Windows generated an error report with the following parameters:
Event:VMM20
P1(appName):vmmservice
P2(appVersion):3.0.6040.0
P3(assemblyName):Engine.BitBos
P4(assemblyVer):3.0.6040.0
P5(methodName):M.V.E.B.VMRefresherBase.UpdateDVDDrives
P6(exceptionType):System.NullReferenceException
P7(callstackHash):5b09
.

The cause of the error was a virtual server that had an iso file mounted in its DVD drive.  The iso file was held on a share on the old SCVMM server, which no longer existed.  Setting the DVD drive to no media resolved the error.