[NUI] Fix CA1044

[NUI] Fix CA1044

- https://docs.microsoft.com/en-us/dotnet/fundamentals/code-analysis/quality-rules/ca1044
- Get accessors provide read access to a property and set accessors provide write access. Although it is acceptable and often necessary to have a read-only property, the design guidelines prohibit the use of write-only properties. This is because letting a user set a value and then preventing the user from viewing the value does not provide any security. Also, without read access, the state of shared objects cannot be viewed, which limits their usefulness.