bucket property Null safety
The getter returns null if state restoration is turned off. When state restoration is turned on or off during the lifetime of this mixin (and hence the return value of this getter switches between null and non-null) didToggleBucket is called.
Interacting directly with this bucket is uncommon. However, the bucket may
be injected into the widget tree in the State's
build method using an
UnmanagedRestorationScope. That allows descendants to claim child
buckets from this bucket for their own restoration needs.
RestorationBucket? get bucket => _bucket;