MakeTabViewModela public type too.
Obviously, it doesn’t make sense for a public property on a public containing-type to be of a type that is not public. How could the property present itself to external assemblies?
Your second sample works because, as a general rule, providing no accessibility modifiers means that the least applicable modifier is chosen as the default – in this case: private. Clearly, there are no consistency issues with declaring a private property of an internal (?) type.