Object reference not set to an instance of an object migrating to 6.4.1

Hi,

We have encountered the following problem while trying to migrate from Mendix version 5.21.2 to version 6.4.1

    System.NullReferenceException: Object reference not set to an instance of an object.
   at Mendix.Modeler.Integration.ConvertChoiceElementOccurrence.FixChoiceElementOccurence(IObjectMappingElement objectMappingElement, IElement rootElement) in C:\jenkins\workspace\AppStudio-Integration-Modeler\src\modeler\Mendix.Modeler.Integration\Conversions\ConvertChoiceElementOccurrence.cs:line 42
   at Mendix.Modeler.Integration.ConvertChoiceElementOccurrence.FixChoiceElementOccurence(IObjectMappingElement objectMappingElement, IElement rootElement) in C:\jenkins\workspace\AppStudio-Integration-Modeler\src\modeler\Mendix.Modeler.Integration\Conversions\ConvertChoiceElementOccurrence.cs:line 49
   at Mendix.Modeler.Integration.ConvertChoiceElementOccurrence.PerformConvert(IMappingDocument mappingDocument) in C:\jenkins\workspace\AppStudio-Integration-Modeler\src\modeler\Mendix.Modeler.Integration\Conversions\ConvertChoiceElementOccurrence.cs:line 33
   at Mendix.Modeler.Integration.ConvertChoiceElementOccurrence.Convert(IProject project) in C:\jenkins\workspace\AppStudio-Integration-Modeler\src\modeler\Mendix.Modeler.Integration\Conversions\ConvertChoiceElementOccurrence.cs:line 24
   at Mendix.Modeler.Projects.Conversions.ProjectConversion.PerformOneTimeConversion(IOneTimeConversion oneTimeConversion) in C:\jenkins\workspace\AppStudio-Modeler\src\Mendix.Modeler.Core\Projects\Conversions\ProjectConversion.cs:line 53
   at Mendix.Modeler.Projects.Conversions.ProjectConversion.DoOneTimeConversions() in C:\jenkins\workspace\AppStudio-Modeler\src\Mendix.Modeler.Core\Projects\Conversions\ProjectConversion.cs:line 37
   at Mendix.Modeler.Projects.Project.<Convert>b__116_0() in C:\jenkins\workspace\AppStudio-Modeler\src\Mendix.Modeler.Core\Projects\Project.cs:line 245
   at Mendix.Modeler.Projects.Project.HackProjectInstance[T](Func`1 func) in C:\jenkins\workspace\AppStudio-Modeler\src\Mendix.Modeler.Core\Projects\Project.cs:line 279
   at Mendix.Modeler.Projects.Project.Convert() in C:\jenkins\workspace\AppStudio-Modeler\src\Mendix.Modeler.Core\Projects\Project.cs:line 243
   at Mendix.Modeler.Storage.Operations.UnitLoader.<>c__DisplayClass18_0.<IsConversionNeeded>b__0(IStorageObject o) in C:\jenkins\workspace\AppStudio-Modeler\src\Mendix.Modeler.Storage\Operations\UnitLoader.cs:line 242
   at Mendix.Modeler.Storage.Operations.TypeWalk.ForEachObjectDo(IStorageObject root, Func`2 work, Boolean skipExcludedObjects) in C:\jenkins\workspace\AppStudio-Modeler\src\Mendix.Modeler.Storage\Operations\TypeWalk.cs:line 87
   at Mendix.Modeler.Storage.Operations.UnitLoader.IsConversionNeeded(IStorageUnit rootUnit) in C:\jenkins\workspace\AppStudio-Modeler\src\Mendix.Modeler.Storage\Operations\UnitLoader.cs:line 240
   at Mendix.Modeler.Storage.Operations.UnitLoader.Load[T](IProgressInfo info, Dictionary`2 _knownUnits, Boolean& conversionNeeded) in C:\jenkins\workspace\AppStudio-Modeler\src\Mendix.Modeler.Storage\Operations\UnitLoader.cs:line 48
   at Mendix.Modeler.Operations.Loader30.<>c__DisplayClass0_0.<DoLoad>b__0(Database database) in C:\jenkins\workspace\AppStudio-Modeler\src\Mendix.Modeler.Core\Operations\Loader30.cs:line 22
   at Mendix.Modeler.Utility.DbConnectors.DbConnector.WithoutTransactionDo[TResult](Func`1 action) in C:\jenkins\workspace\AppStudio-Modeler\src\Mendix.Modeler.Utility\DbConnectors\DbConnector.cs:line 64
   at Mendix.Modeler.Utility.DbConnectors.SQLiteConnector.Do[TResult](String filePath, Boolean useTransaction, Func`2 action) in C:\jenkins\workspace\AppStudio-Modeler\src\Mendix.Modeler.Utility\DbConnectors\SQLiteConnector.cs:line 58
   at Mendix.Modeler.Storage.Database.Do[TResult](String filePath, Boolean useTransaction, Func`2 action) in C:\jenkins\workspace\AppStudio-Modeler\src\Mendix.Modeler.Storage\Database.cs:line 0
   at Mendix.Modeler.Operations.Loader30.DoLoad(IProgressInfo info, String filePath) in C:\jenkins\workspace\AppStudio-Modeler\src\Mendix.Modeler.Core\Operations\Loader30.cs:line 18

We have removed all the excluded documents. Can someone please assist us with this issue?