FileSystemException (OS Error: Is a directory, errno = 21)

I was starting my app in flight mode (no internet connection) to check if cached network images would reload automatically when I turned internet access back on. The urls are stored in firestore, so not sure if the urls were cached or not.

(Firestore does automatically update data when internet is turned back on, but cached network images are only displayed when their widgets are reloaded)

When turning internet back on this exception happened:
```
I/flutter (10496): ══╡ EXCEPTION CAUGHT BY IMAGE RESOURCE SERVICE ╞════════════════════════════════════════════════════
I/flutter (10496): The following FileSystemException was thrown resolving an image codec:
I/flutter (10496): Cannot open file, path = '/data/user/0/se.myapp/cache/libCachedImageData' (OS Error: Is
I/flutter (10496): a directory, errno = 21)
I/flutter (10496):
I/flutter (10496): When the exception was thrown, this was the stack:
I/flutter (10496): #0 _File.open. (dart:io/file_impl.dart:364:9)
I/flutter (10496): (elided 13 frames from package dart:async)
I/flutter (10496): ...
I/flutter (10496):
I/flutter (10496): Path: /data/user/0/se.myapp/cache/libCachedImageData
I/flutter (10496): ════════════════════════════════════════════════════════════════════════════════════════════════════
```