Fix undefined login error when updating active pull request view

[Copilot]

When switching accounts, getCurrentUser() can return undefined during the transition, causing "Cannot read properties of undefined (reading 'login')" when accessing currentUser.login or pullRequest.author.login.

Changes

• Add null guard for currentUser after async resolution
• Add null guard for pullRequest.author before property access

if (!currentUser) {
    throw new Error('Unable to get current user');
}

if (!pullRequest.author) {
    throw new Error('Pull request author is missing');
}

Follows the existing pattern used for the !pullRequest check. Provides clearer error messages instead of the cryptic property access error.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Error updating active pull request view: Cannot read properties of undefined (reading 'login')</issue_title>
<issue_description>1. Log out from your corp account
2. Login with your free account
3. Sometimes I see the Error updating active pull request view: Cannot read properties of undefined (reading 'login')

</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Error updating active pull request view: Cannot read properties of undefined (reading 'login') #8187

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