Error building Player: NullReferenceException

Error building Player: NullReferenceException: Object reference not set to an instance of an object

I get this error while trying to build an Android app. There is no additional information. How do I even begin debugging this?

Can you give some more information? For what I see now is: Do you have the Android SDK located? You can find it here: Unity>Preferences>External Tools.

~Benjamin