Fixed a null error on "do not show again" warning messages

[michaelWuensch]

Description

This PR fixes an error, that occured on UserGuardian messages which have been flagged as "do not show again" and have no callback action. One example is the new "old lnd" warning. Having this message flagged as "do not show again" and still not upgrading could lead to the wallet not loading.

Motivation and Context

Getting rid of bugs.

How Has This Been Tested?

Regtest with LND 10.0

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the Contribution document.
• [ ] I have added tests to cover my changes.
• [x] All new and existing tests passed.