fix: new ArenaTopics() leaves every segment null, so topic segment access throws #192

Runtime/ArenaTopics.cs:29 declares

public readonly struct ArenaTopics

and its constructor at :39-62 takes ten arguments that all default to "". That is not the parameterless constructor: new ArenaTopics() (and default(ArenaTopics)) zero-initializes the struct, so REALM, nameSpace, sceneName … (:63-72) come out null, not empty string. The default arguments are unreachable through that syntax.

The interpolated getters (:78-108) still produce well-shaped strings, because $"…" renders a null as empty — which is exactly what hides the problem. Any caller touching a segment directly gets a NullReferenceException from a value that looks default-constructed.

Observed

Runtime/ArenaTopics.cs compile-linked and run on .NET 8:

new ArenaTopics():
  REALM==null: True, nameSpace==null: True, sceneName==null: True, idTag==null: True

new ArenaTopics().PUB_SCENE_OBJECTS   => "/s///o//"        (renders fine, hides the nulls)
new ArenaTopics().nameSpace.Length    => System.NullReferenceException
new ArenaTopics().sceneName.StartsWith("x") => System.NullReferenceException

new ArenaTopics("").nameSpace.Length  => 0                 (the ctor path is fine)

So it is specifically the parameterless path that is unsafe; passing even one argument runs the constructor and the remaining defaults apply.

Fix options

• coalesce in the property getters (get { return _nameSpace ?? ""; }), which fixes default(ArenaTopics) too — the only option that closes the hole completely for a struct;
• make the type a class, so there is a real parameterless constructor to run;
• or keep it as-is, add a static ArenaTopics.Empty built through the constructor, and document that new ArenaTopics() is not it.

#183 pins the current behaviour in Tests~/DotNet/ArenaUnity.PureTests/ArenaTopicsTests.cs with three Is.Null assertions; those flip if the coalescing option is chosen.

Generated by Claude Code