Closed Issue: NullReferenceException when using Migrations and 6.0 alpha3 [914]

I just updated my project to the 6.0.0-alpha 3 and now I am getting the following exception:

```
[NullReferenceException: Der Objektverweis wurde nicht auf eine Objektinstanz festgelegt.]
System.Data.Entity.Edm.Serialization.EdmSerializationVisitor.VisitEdmAssociationSet(AssociationSet item) +182
System.Data.Entity.Edm.EdmModelVisitor.VisitCollection(IEnumerable`1 collection, Action`1 visitMethod) +141
System.Data.Entity.Edm.Serialization.EdmSerializationVisitor.VisitEdmEntityContainer(EntityContainer item) +355
System.Data.Entity.Edm.EdmModelVisitor.VisitCollection(IEnumerable`1 collection, Action`1 visitMethod) +141
System.Data.Entity.Edm.Serialization.EdmSerializationVisitor.Visit(EdmModel edmModel, String namespaceName, String provider, String providerManifestToken) +57
System.Data.Entity.Edm.Serialization.EdmSerializationVisitor.Visit(EdmModel edmModel, String provider, String providerManifestToken) +268
System.Data.Entity.Edm.Serialization.SsdlSerializer.Serialize(EdmModel dbDatabase, String provider, String providerManifestToken, XmlWriter xmlWriter, Boolean serializeDefaultNullability) +211
System.Data.Entity.ModelConfiguration.Edm.Serialization.EdmxSerializer.WriteEdmxRuntime() +463
System.Data.Entity.ModelConfiguration.Edm.Serialization.EdmxSerializer.Serialize(DbDatabaseMapping databaseMapping, DbProviderInfo providerInfo, XmlWriter xmlWriter) +445
System.Data.Entity.Utilities.DbContextExtensions.GetModel(Action`1 writeXml) +174
System.Data.Entity.Migrations.DbMigrator..ctor(DbMigrationsConfiguration configuration, DbContext usersContext) +486
System.Data.Entity.MigrateDatabaseToLatestVersion`2.InitializeDatabase(TContext context) +65
System.Data.Entity.Internal.InternalContext.PerformInitializationAction(Action action) +65
System.Data.Entity.Internal.InternalContext.PerformDatabaseInitialization() +579
System.Data.Entity.Internal.RetryAction`1.PerformAction(TInput input) +208
System.Data.Entity.Internal.LazyInternalContext.InitializeDatabaseAction(Action`1 action) +235
```

Going back down to 6.0.0-alpha2 does not throw this exception and works fine for me.
This happens to me on the second context I initialize in the same DB using Migration. When I disable Migrations it works fine (but of course my DB is not updated). I am not shure if the second context has something EF does not like or if it is a general problem when using two DBContext's for the same DB and Migrations.

Comments: Duplicate of 867 - http://entityframework.codeplex.com/workitem/867