Safeguard dialog.close call with optional chaining to prevent potential runtime errors

[cstns]

Description

Was able to replicate the above on prod following the changes in the previous pr.

Safeguarding the catch route in order to pass the custom error through.

Related Issue(s)

follow up on #6798
part of #6797

Checklist

• I have read the contribution guidelines
• Suitable unit/system level tests have been added and they pass
• Documentation has been updated
  • Upgrade instructions
  • Configuration details
  • Concepts
• Changes flowforge.yml?
  • Issue/PR raised on FlowFuse/helm to update ConfigMap Template
  • Issue/PR raised on FlowFuse/CloudProject to update values for Staging/Production
• Link to Changelog Entry PR, or note why one is not needed.

Labels

• Includes a DB migration? -> add the area:migration label

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.60%. Comparing base (5a6825f) to head (9d1c119).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6808   +/-   ##
=======================================
  Coverage   76.60%   76.60%           
=======================================
  Files         398      398           
  Lines       20122    20122           
  Branches     4844     4844           
=======================================
  Hits        15414    15414           
  Misses       4708     4708           

Flag	Coverage Δ
backend	76.60% <ø> (ø)

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