fix: initialize user_can_view before conditional check

[thecaptain789]

Summary

Fixes UnboundLocalError when the @cl.on_shared_thread_view hook is not defined.

Problem

In backend/chainlit/server.py, the user_can_view variable was only assigned inside the optional on_shared_thread_view hook block, but was used unconditionally in the subsequent check at line 1019:

if (not user_can_view) and (not is_shared):

When the hook is not defined/registered, this caused:

UnboundLocalError: cannot access local variable 'user_can_view' where it is not associated with a value

Solution

Initialize user_can_view = False before the conditional block, ensuring the variable is always defined regardless of whether the hook exists.

Changes

+    user_can_view = False
     if getattr(config.code, "on_shared_thread_view", None):

Testing

1. Create a minimal Chainlit app without defining @cl.on_shared_thread_view
2. Start the server
3. Access a shared thread URL
4. Verify no UnboundLocalError is raised

Related Issue

Fixes #2766

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Summary by cubic

Initialize user_can_view to False before the shared-thread access check to prevent UnboundLocalError when @cl.on_shared_thread_view is not defined. This ensures apps without the hook handle shared thread views without crashing.

^{Written for commit 5e5e00c. Summary will update on new commits.}

[cubic-dev-ai]

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