This PR is trying to fix issues #17798.

Root Cause

The error message construction used it->second unconditionally, but the Verify() condition was:

• it == currently_defined_.end() || redefine_is_allowed

This means when the condition evaluates to false (triggering the error), it could be due to:

1. it != end() && !redefine_is_allowed => Safe to access it->second
2. it == end() && !redefine_is_allowed => Invalid to access it->second

Solution

The fix ensures safe iterator access by:

1. Storing the Verify result: Instead of chaining error messages directly to Verify(), store the result in a variable
2. Conditional dereferencing: Only access it->second when it != end()
3. Meaningful error messages: Provide appropriate messages for both cases