OpsHub: ERROR Object reference not set to an insta

We're checking into the option of moving to Visual Studio Online from our on premises TFS. Running the migration tool, it shows this error during validation:

ERROR Object reference not set to an instance of an object.

I'm trying to migrate to a new setup on Visual Studio Online from our TFS 2013 server using version 1.2.0 of the migration tool.

Here is the full log entry:

2015-05-12 13:21:40,383 [1] ERROR Object reference not set to an instance of an object.
System.NullReferenceException: Object reference not set to an instance of an object.
   at TFSMigrationUI.ViewModel.MigrationSummeryViewModel.worker_RunWorkerCompleted(Object sender, RunWorkerCompletedEventArgs e) in e:\OVSMUBranch\TFSMigrationUI\ViewModel\MigrationSummeryViewModel.cs:line 928
   at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
   at MS.Internal.Threading.ExceptionFilterHelper.TryCatchWhen(Object source, Delegate method, Object args, Int32 numArgs, Delegate catchHandler)

Any help would be greatly appreciated.

The Migration Utility expects "Project Collection Service Accounts" to be present in the VSO account Groups which is created by default when you create your VSO account, but due to some unknown issue some accounts created between the specific date this group was created as "Team Foundation Service Accounts" as in your case, so the migration utility was not able to find the desired group which resulted in the issue you are facing, you can continue the migration by creating a VSO group "Project Collection Service Accounts", add the user in this group which was used for authentication of VSO account for migration utility and make sure the newly created VSO group is a member of "Team Foundation Administrators" so you won't encounter any permission related issue.