This was odd. I received this error after making a second inner class within my custom constructor. I googled the error and read this, which lead me to try the suggestion: remove the methods of the outer class and save. This did not work, so I removed the methods and member variables of the outer class and hit save. This didn't work, so I removed the methods, member variables, and the other inner class from the custom controller, leaving only the second inner class which caused the error, and it saved without error. Pasting everything I just deleted back in the class and saving also didn't cause an error and everything worked.