TfsWorkItemMigrationProcessor PopulateWorkItem ClosedDate issues

[anderssonpof]

I have encountered an issue that is puzzling me.

One work item type I'm having issues with cannot seems to populate fields correctly.
So I thought I would move this type to 'Bug' instead because the type i'm migrating is defunct anyway and its only for history reasons they are copied.

However I'm receiving the following error.

Microsoft.TeamFoundation.WorkItemTracking.Client.FieldDefinitionNotExistException: TF26027: A field definition Microsoft.VSTS.Common.ClosedDate in the work item type definition file does not exist. Add a definition for this field or remove the reference to the field and try again.
   at Microsoft.TeamFoundation.WorkItemTracking.Client.FieldDefinitionCollection.get_Item(String name)
   at Microsoft.TeamFoundation.WorkItemTracking.Client.FieldCollection.get_Item(String name)
   at MigrationTools.Processors.TfsWorkItemMigrationProcessor.PopulateWorkItem(WorkItemData oldWorkItemData, WorkItemData newWorkItemData, String destType) in D:\a\azure-devops-migration-tools\azure-devops-migration-tools\src\MigrationTools.Clients.TfsObjectModel\Processors\TfsWorkItemMigrationProcessor.cs:line 422
   at MigrationTools.Processors.TfsWorkItemMigrationProcessor.ReplayRevisions(List`1 revisionsToMigrate, WorkItemData sourceWorkItem, WorkItemData targetWorkItem) in D:\a\azure-devops-migration-tools\azure-devops-migration-tools\src\MigrationTools.Clients.TfsObjectModel\Processors\TfsWorkItemMigrationProcessor.cs:line 767

If I debug the code I get to PopulateWorkItem and this particual code regarding ClosedDate

newWorkItem.Title = oldWorkItem.Title;
if (newWorkItem.Fields.Contains("Microsoft.VSTS.Common.ClosedDate") && newWorkItem.Fields["Microsoft.VSTS.Common.ClosedDate"].IsEditable)
{
    newWorkItem.Fields["Microsoft.VSTS.Common.ClosedDate"].Value = oldWorkItem.Fields["Microsoft.VSTS.Common.ClosedDate"].Value;
}
newWorkItem.State = oldWorkItem.State;
if (newWorkItem.Fields.Contains("Microsoft.VSTS.Common.ClosedDate") && newWorkItem.Fields["Microsoft.VSTS.Common.ClosedDate"].IsEditable)
{
    newWorkItem.Fields["Microsoft.VSTS.Common.ClosedDate"].Value = oldWorkItem.Fields["Microsoft.VSTS.Common.ClosedDate"].Value;
}
newWorkItem.Reason = oldWorkItem.Reason;

The first if statement is true but the second get false.
While (newWorkItem.Fields.Contains("Microsoft.VSTS.Common.ClosedDate") is true newWorkItem.Fields["Microsoft.VSTS.Common.ClosedDate"].IsEditable) is false when running the second time.

The second if statement seems to be redundant at first glance?