TopicsMobileHeader crashes when topic is null #15366

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Overview

On small screens, TopicsPage renders TopicsMobileHeader before topic has loaded, and the header's render throws a TypeError on topic.title.

Complexity: Low
Target branch: develop

Context

Production, 90 days to 2026-09-24:

TypeError: null is not an object (evaluating 'this.topic.title')

• TopicsMobileHeader.vue L13-L22 reads topic.title and topic.thumbnail with no null check.
• TopicsPage initialises topic to null (L364). TopicsHeader guards each field (L37-L39), but TopicsMobileHeader gets topic unguarded (L60-L63).

The Change

• TopicsMobileHeader should handle a null topic the way TopicsHeader does, until the topic loads.

Acceptance Criteria

• TopicsMobileHeader renders without error when topic is null.
• Once topic loads, the header shows its title and thumbnail as it does today.
• A Jest test renders TopicsMobileHeader with topic: null.

AI usage

I picked this from a Sentry triage with Claude Code. Claude Code queried Sentry, traced the code and drafted the sections.