I can tell you what must have happened with you is that you must have forgotten to register your controllers explicitly in the container. Now the Unity will try to fix the unregistered concrete types for you but as it is unable to resolve it so it will return null. Now the Web API will try to create your controller itself but as it does not have the default constructor in this case it will throw the above error listed by you.
This error message is certainly misleading for beginners like you and it fail to explain you the real cause.
This is the configuration error and it comes because of adding the second constructor to the
DbContext. Now the Unity will try to pick a constructor which is having the most arguments, but here it has no idea how to fix this particular constructor.
It seems you are trying to use a Unity's auto wiring capabilities simply to create a
DbContext. I must tell you the
DbContext is the most special type and so it shouldn't be auto wired. As it is the framework type so you should fallback to the registering it using the factory delegate.