Move some code around to prevent possible error #18395

Description:

fixes #18393

Review

• [ ] Functional review done
• [ ] Potential edge cases thought about (behavior of the code with strange input, with strange internal state or possible interactions with other Matomo subsystems)
• [ ] Usability review done (is anything maybe unclear or think about anything that would cause people to reach out to support)
• [ ] Security review done
• [ ] Wording review done
• [ ] Code review done
• [ ] Tests were added if useful/possible
• [ ] Reviewed for breaking changes
• [ ] Developer changelog updated if needed
• [ ] Documentation added if needed
• [ ] Existing documentation updated if needed

The period validator should actually throw an error in isPeriodAllowedForUI in case an unexpected value is provided. If we use string in the getRequestVar method, that method would already throw an exception, that the type doesn't fit, but that message might be not that accurate.