Zeroing weak references are by definition nullable, and adding nonnull or _Nonnull yields a mutual-exclusivity error. However, when -Wnullability-completeness is enabled, in non-audited header regions it's necessary to add the nullable property keyword to avoid a warning. It should not be necessary to add code clutter to restate the nullability of a weak property even under -Wnullability-completeness.
Additionally, the fix-it hints are both non-idiomatic Objective-C (adding _Nullable to the property's pointer type rather than in the @property attributes) and suggest the option of adding _Nonnull (which would be an error).