Handle case where town has no governor assigned (governor = null)

[georgyrudnev]

PR Checklist

Please check if your PR fulfills the following requirements:

• All new classes have class-level documentation comments, if there are any at all
• Tests for the changes have been added (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation update
• Other... Please describe:

What is the current behavior?

Currently there is sometimes a null exception or error when a null value is assigned to the governor in the change town governor handler.

What is the new behavior?

Resolves #

Null value is now handled as well.

Other information