The `[NullAllowed]` attribute should not be allowed on methods but it
could break existing binding projects.
Historically it was used on property setters. However using the attribute
on _other_ methods can be misleading, e.g. should it apply to all
parameters, the return value... and its presence/action can be
misinterpreted in code reviews leading to binding bugs.
reference: https://github.com/xamarin/xamarin-macios/issues/5416
Rolf Bjarne Kvinge
Sebastien Pouliot