local variable 'arr' referenced before assignment due to list of lists

[sgbaird]

The PIL images from imagen-pytorch were returning this error during xc.png2xtal(images). Not sure if this is something wrong on my side or the imagen-pytorch side.

[[<PIL.Image.Image image mode=L size=64x64 at 0x7F417391E9D0>],
 [<PIL.Image.Image image mode=L size=64x64 at 0x7F42CBAF9F90>],
 [<PIL.Image.Image image mode=L size=64x64 at 0x7F42C4FD9050>],
 [<PIL.Image.Image image mode=L size=64x64 at 0x7F42C4FD9210>],
 [<PIL.Image.Image image mode=L size=64x64 at 0x7F42CBBBDF10>],
 [<PIL.Image.Image image mode=L size=64x64 at 0x7F42CBBBDC90>],
 [<PIL.Image.Image image mode=L size=64x64 at 0x7F42CBBBDED0>],
 [<PIL.Image.Image image mode=L size=64x64 at 0x7F42CBBBDE90>],
 [<PIL.Image.Image image mode=L size=64x64 at 0x7F42C515D590>],
 [<PIL.Image.Image image mode=L size=64x64 at 0x7F417391E410>]]

---------------------------------------------------------------------------
UnboundLocalError                         Traceback (most recent call last)
[<ipython-input-29-5af3e9711054>](https://localhost:8080/#) in <module>
----> 1 gen_structures = xc.png2xtal(gen_images)
      2 mptm.evaluate_and_record(fold, gen_structures)
      3 print(mptm.recorded_metrics)

[/usr/local/lib/python3.7/dist-packages/xtal2png/core.py](https://localhost:8080/#) in png2xtal(self, images, save)
    569             if mode == "RGB":
    570                 arr = arr.transpose(2, 0, 1)
--> 571             data_tmp.append(arr)
    572 
    573         data = np.stack(data_tmp, axis=0)

UnboundLocalError: local variable 'arr' referenced before assignment

The issue was that I had a list of lists instead of just a list of PIL objects. Not a very helpful error, but maybe a type hint would have revealed this issue. Not sure if I have the right type hint for that.