Database not connected 'NoneType' object has no attribute 'txn'

[romeshshil]

In [multi-tenancy](https://dgraph.io/docs/enterprise-features/multitenancy/) environments, PyDgraph provides a new method login_into_namespace(), which will allow the users to login to a specific namespace.
client_stub = pydgraph.DgraphClientStub('localhost:9080')
client = pydgraph.DgraphClient(client_stub)
// Login to namespace groot user of namespace 123
client.login_into_namespace("groot", "password", "123")

Get Error:

Exception Type: | AttributeError
Exception Value: 'NoneType' object has no attribute 'txn'

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