Custom Units Checker qualifiers need to be supertypes of @UnitsBottom

Problem report: https://groups.google.com/d/msg/checker-framework-discuss/M0tPOCT9RGA/aRETL3X3CQAJ

@UnitsBottom is the implicit qualifier for null: @ImplicitFor(typeNames = Void.class, literals = LiteralKind.NULL).

For the built-in qualifiers, assigning null works, e.g. @m Float x = null;

However, for a custom units annotation, the same is not true and an error is raised:

error: [return.type.incompatible] incompatible types in return.
            return null;
                   ^
 found   : @UnknownUnits  null
 required: @frac(Prefix.deci) Float

Note that @UnitsBottom has the following meta-annotation: @SubtypeOf({}) // needs to be done programmatically
So most likely the error is that setting the correct supertypes only considered the built-in units and incorrectly ignores costum units annotations.