MaxScale crashes (SIGABRT) with std::logic_error when performing a backup operation via MaxGUI Backup Operations panel

Summary of issue:
Prospect POC - MaxScale crashes with a fatal signal 6 (SIGABRT) when a backup operation is
triggered via the Backup Operations panel in MaxGUI. Confirmed reproducible,
occurred twice on two separate attempts approximately 2 hours apart, with an
identical stack trace both times.

Steps to reproduce:
1. Open MaxGUI > Backup Operations
2. Select a source server and enter a backup name
3. Proceed through the wizard to trigger Create Backup
4. MaxScale crashes and automatically restarts shortly after

Actual result:
MaxScale terminates with fatal signal 6. Exception thrown:

terminate called after throwing an instance of 'std::logic_error'
what(): basic_string: construction from null is not valid

Stack trace shows the crash occurring in:
mon_op::BackupOperation::check_directory_entries
(server/modules/monitor/mariadbmon/monitor_commands.cc:3200)

This is called with an already-established SSH session
(std::shared_ptr<ssh::Session>), so the SSH connection itself succeeds; the
crash occurs while processing the results of a directory listing on the
target node, not while establishing the connection.

Expected result:
Backup operation should complete, or fail gracefully with a handled error,
rather than crashing the MaxScale process.

Additional notes:

• Reproducible on two independent attempts, identical stack trace both times.
• Full log file and screenshot of the MaxGUI wizard state attached.
• Customer has a workaround in place (running mariadb-backup directly on the
  database nodes), so this is not currently blocking their POC, but the
  MaxGUI Backup Operations feature itself is unusable in this environment.
• Related customer support ticket: #252526