The current pipeline code used to set a base for a tree object
that might or might not exist. Depending on it it would either
use that object or reset its base. Avoid doing that because it
prohibits us from properly interpreting the `id` of an object
if the latter is also set when `base_id` is assigned, since
that base might not exist and thus the `id` would not actually
mean that the the contents of tree associated with the object.
Therefore we use `ObjectStore.get` and return the result if it
is not None or a fresh Object otherwise.
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