BUG: do not fail when stack()ing unsortable level
- [x] closes #18310
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18363?src=pr&el=h1) Report
> Merging [#18363](https://codecov.io/gh/pandas-dev/pandas/pull/18363?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/f724066f4b3e8b402f7307a132b0d9c5e89ab60f?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `77.77%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18363/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/18363?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18363      +/-   ##
==========================================
- Coverage   91.38%   91.36%   -0.03%     
==========================================
  Files         164      164              
  Lines       49790    49790              
==========================================
- Hits        45501    45490      -11     
- Misses       4289     4300      +11
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.16% <77.77%> (-0.01%)` | :arrow_down: |
| #single | `39.53% <0%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18363?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/multi.py](https://codecov.io/gh/pandas-dev/pandas/pull/18363?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9tdWx0aS5weQ==) | `96.21% <77.77%> (-0.17%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18363?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18363?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.8% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18363?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18363?src=pr&el=footer). Last update [f724066...c0a6136](https://codecov.io/gh/pandas-dev/pandas/pull/18363?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18363?src=pr&el=h1) Report
> Merging [#18363](https://codecov.io/gh/pandas-dev/pandas/pull/18363?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/f724066f4b3e8b402f7307a132b0d9c5e89ab60f?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `77.77%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18363/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/18363?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18363      +/-   ##
==========================================
- Coverage   91.38%   91.36%   -0.03%     
==========================================
  Files         164      164              
  Lines       49790    49790              
==========================================
- Hits        45501    45490      -11     
- Misses       4289     4300      +11
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.16% <77.77%> (-0.01%)` | :arrow_down: |
| #single | `39.53% <0%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18363?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/multi.py](https://codecov.io/gh/pandas-dev/pandas/pull/18363?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9tdWx0aS5weQ==) | `96.21% <77.77%> (-0.17%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18363?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18363?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.8% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18363?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18363?src=pr&el=footer). Last update [f724066...c0a6136](https://codecov.io/gh/pandas-dev/pandas/pull/18363?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18363?src=pr&el=h1) Report
> Merging [#18363](https://codecov.io/gh/pandas-dev/pandas/pull/18363?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/b00e62c273cd60739b538314156b65f0f79350f9?src=pr&el=desc) will **decrease** coverage by `0.04%`.
> The diff coverage is `77.77%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18363/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/18363?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18363      +/-   ##
==========================================
- Coverage   91.38%   91.33%   -0.05%     
==========================================
  Files         164      164              
  Lines       49790    49790              
==========================================
- Hits        45501    45478      -23     
- Misses       4289     4312      +23
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.14% <77.77%> (-0.03%)` | :arrow_down: |
| #single | `39.53% <0%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18363?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/multi.py](https://codecov.io/gh/pandas-dev/pandas/pull/18363?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9tdWx0aS5weQ==) | `96.21% <77.77%> (-0.17%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18363?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/18363?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `63.44% <0%> (-1.82%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18363?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.8% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18363?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18363?src=pr&el=footer). Last update [b00e62c...6d267a2](https://codecov.io/gh/pandas-dev/pandas/pull/18363?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @jreback are you waiting for any action from me on this one? nope!