app crashes when trying to save

This is probably not flatpak specific, but let me file it here, as I am experiencing it with the flatpaked version.

```
Traceback (most recent call last):
File "/app/lib/python3.7/site-packages/piexif/_dump.py", line 272, in _value_to_bytes
value_str = raw_value + b"\x00" * (4 - length)
TypeError: can only concatenate tuple (not "bytes") to tuple

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/app/lib/python3.7/site-packages/piexif/_dump.py", line 337, in _dict_to_bytes
offset)
File "/app/lib/python3.7/site-packages/piexif/_dump.py", line 274, in _value_to_bytes
raise ValueError("Got invalid type to convert.")
ValueError: Got invalid type to convert.

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/app/bin/main.py", line 1137, in save
picture.export()
File "/app/bin/main.py", line 481, in export
exif_bytes = piexif.dump(self.exif)
File "/app/lib/python3.7/site-packages/piexif/_dump.py", line 68, in dump
exif_set = _dict_to_bytes(exif_ifd, "Exif", zeroth_length)
File "/app/lib/python3.7/site-packages/piexif/_dump.py", line 341, in _dict_to_bytes
'{} in {} IFD. Got as {}.'.format(key, ifd, type(ifd_dict[key]))
ValueError: "dump" got wrong type of exif value.
41729 in Exif IFD. Got as .
```