Assert usage in dart code

[donpui]

We have several places where we use assert in dart flutter code:

destiny/lib/views/shared/file_picker.dart

Line 107 in cdc4b69

assert(result?.files.length == 1);

All asserts: https://github.com/LeastAuthority/destiny/search?q=assert%28

If file is not selected (User cancel selection file), so this statement becomes false: assert(result?.files.length == 1);

Not fully sure if we should use here assert and just rewrite exception handling.

To may understanding, we have not enabled asserts in release mode.