BUG: crash in df.groupby.rolling.mean with forward window indexer
- [ ] I have checked that this issue has not already been reported.

- [ ] I have confirmed this bug exists on the latest version of pandas.

- [ ] (optional) I have confirmed this bug exists on the master branch of pandas.

---

**Note**: Please read [this guide](https://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports) detailing how to provide the necessary information for us to reproduce your bug.

#### Code Sample, a copy-pastable example

```python
# Your code here

Here is a replicate code:
df = pd.DataFrame({'B': [np.nan, 1, 2, np.nan, 4,5,6,7,8,9,10,11,12,13,14,15]})
df['A']=1
indexer = pd.api.indexers.FixedForwardWindowIndexer(window_size=12) # forward 12 period
# First time is OK
df['mean']=df.groupby('A')['B'].rolling(window=indexer, min_periods=1).mean().reset_index().loc[:,'B']
# Second time would crash!
df['mean']=df.groupby('A')['B'].rolling(window=indexer, min_periods=1).mean().reset_index().loc[:,'B']

If don't have groupby, it won't crash.
If don't forward window indexer, it won't crash.
If in the first time, it won't crash.


```

#### Problem description

[this should explain **why** the current behaviour is a problem and why the expected output is a better solution]

When you use df.groupby.rolling.mean with a forward-looking period, it would crash if you run the df.groupby.rolling.mean again.



#### Expected Output

#### Output of ``pd.show_versions()``

<details>
INSTALLED VERSIONS
------------------
commit           : c7f7443c1bad8262358114d5e88cd9c8a308e8aa
python           : 3.8.11.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
Version          : 10.0.19042
machine          : AMD64
processor        : Intel64 Family 6 Model 165 Stepping 2, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : zh_CN
LOCALE           : Chinese (Simplified)_China.936

pandas           : 1.3.1
numpy            : 1.20.3
pytz             : 2021.1
dateutil         : 2.8.2
pip              : 21.2.2
setuptools       : 52.0.0.post20210125
Cython           : 0.29.24
pytest           : 6.2.4
hypothesis       : None
sphinx           : 4.0.2
blosc            : None
feather          : None
xlsxwriter       : 3.0.1
lxml.etree       : 4.6.3
html5lib         : 1.1
pymysql          : None
psycopg2         : None
jinja2           : 2.11.3
IPython          : 7.26.0
pandas_datareader: None
bs4              : 4.9.3
bottleneck       : 1.3.2
fsspec           : 2021.07.0
fastparquet      : None
gcsfs            : None
matplotlib       : 3.4.2
numexpr          : 2.7.3
odfpy            : None
openpyxl         : 3.0.7
pandas_gbq       : None
pyarrow          : None
pyxlsb           : None
s3fs             : None
scipy            : 1.6.2
sqlalchemy       : 1.4.22
tables           : 3.6.1
tabulate         : None
xarray           : None
xlrd             : 2.0.1
xlwt             : 1.3.0
numba            : 0.53.1
[paste the output of ``pd.show_versions()`` here leaving a blank line after the details tag]

</details>

![image](https://user-images.githubusercontent.com/31037879/131203874-5ba9b4ca-0377-46b7-be16-a08faa970f62.png)

