Tier config with invalid tenant fails on rebalance with NPE

If a tier config is added with an invalid tenant, the table config update is succeeds but a rebalance of the table returns with an NPE. Then to find the exact error the controller logs need to be searched.

Is that intended as a tenant may not always be present? Or should adding a tier config that contains a tag which does not point to any instance fail when updating the table config?

Start by reproducing the table config update with a tier config containing an invalid tenant, then trigger a table rebalance. Inspect the controller logs alongside the rebalance result to determine the expected handling; done means the behavior for a missing tenant is explicitly decided and the NPE is no longer the user-visible outcome.

Written by the indexing model from the issue text.