TST/BUG: Categorical dtypes cause error when attempting stacked bar plot
- [x] closes #13019
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13038?src=pr) is **84.07%**

> Merging [#13038](https://codecov.io/gh/pydata/pandas/pull/13038?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will not change coverage

``` diff
@@             master     #13038   diff @@
==========================================
  Files           136        136          
  Lines         49994      49994          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits          42028      42028          
  Misses         7966       7966          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [84725fa...dc57a6c](https://codecov.io/gh/pydata/pandas/compare/84725fa936b4136657f5ea02c588790de6866899...dc57a6c80b26d440381a08e92ffaae24568315f0)
 👍  Not sure if we want to add a release note explicitly stating this was fixed (even if we don't know exactly what PR fixed it)
 thanks!
 whatsnew added here: https://github.com/pydata/pandas/commit/db35ff455227d1b705509966740d3d6607faa3d9
