Safely handle missing active viewport

[Sofien-Sellami]

Context

Sometimes, when switching hanging protocols, the activeViewport can be undefined.
This caused errors because the code assumed that an active viewport would always be present.

Changes & Results

Added a guard clause in _syncWithActiveViewport to safely handle case where:

• activeViewport is undefined.
• Ensures prefetching logic only runs when a valid viewport exists.

Before:

Changing hanging protocols could throw errors when no active viewport was set.

Before:

The app now gracefully skips syncing when there is no active viewport.

Testing

Checklist

PR

• My Pull Request title is descriptive, accurate and follows the
  semantic-release format and guidelines.

Code

• My code has been well-documented (function documentation, inline comments,
  etc.)

Public Documentation Updates

• The documentation page has been updated as necessary for any public API
  additions or removals.

Tested Environment

• OS:
• Node version:
• Browser:

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