Incorrect type-hint in UI layer's `check_valid_ref_id()` function

The function is the following: https://github.com/DiamondLightSource/httomo/blob/7842bae27c1777545279aa36e8c10d9c4eeb489f/httomo/ui_layer.py#L241-L263

Pyright complains about the body of the second `if` statement, saying the following:
```
Type analysis indicates code is unreachable
```

This is understandable, because the type of `method` in the function signature is `MethodWrapper`, and so should never be `None`. Thus, the condition in the `if` statement will never evaluate to `True`, and the body will never be executed.

One way to easily fix this is by making `method` have type `Optional[MethodWrapper]`. However, some investigation into whether it should ever be `None` (and possibly investigation in further refactorings in other logic in the UI layer) may be wise before opting for the simplest fix.