get_columns_in_relation fails on the first run, when called on a model created in that run

[ali-tny]

Describe the bug

Calling get_columns_in_relation on a model that was created in the same run leads to a expected string or bytes-like object error on the first run, where the cache is not populated correctly.

Steps To Reproduce

Create a model and a downstream model that uses get_columns_in_relation, created in the same run.

Expected behavior

No error (falling back to querying information schema where the cache is not sufficient)

Screenshots and log output

Fails with

Unhandled error while executing model_name
expected string or bytes-like object

The output of dbt --version:

installed version: 0.20.0
   latest version: 0.20.0

Up to date!

Plugins:
  - spark: 0.20.0

The operating system you're using: MacOS Mojave

The output of python --version: Python 3.8.3

Additional context

Described in this slack thread

[jtcohen6]

Thanks for opening @ali-tny, and for contributing the fix! I'm out of the office today, will take a look early next week