In the example below, before, we would get the value 'default'
instead of the value `None`.
```
import solara

@solara.component
def Page():
    size = solara.use_reactive(None)
    print("size", repr(size.value))

    with solara.ToggleButtonsSingle(value=size, on_value=size.set):
        solara.Button("x-small", value="x-small")
        solara.Button("small", value="small")
        solara.Button("default", value=None)
        solara.Button("large", value="large")
        solara.Button("x-large", value="x-large")
```