getTags is unstable

[acharlop]

Please see this repo reproducing the issue here

Error that is returned:

Unhandled exception:
NoSuchMethodError: The getter 'single' was called on null.
Receiver: null
Tried calling: single
#0      Object._noSuchMethod (dart:core-patch/object_patch.dart:43)
#1      Object.noSuchMethod (dart:core-patch/object_patch.dart:47)
#2      Tag.parseCatFile (package:git/src/tag.dart:36:41)
#3      GitDir.getTags.<anonymous closure>.<anonymous closure>.<anonymous closure> (package:git/src/git_dir.dart:75:22)