Null check operator used on a null value in csslib/visitor.dart

[mikehearn]

I get a lot of errors like this:

Unhandled exception:
Null check operator used on a null value
#0      Visitor.visitSupportsDirective (package:csslib/visitor.dart:178:19)
#1      SupportsDirective.visit (package:csslib/src/tree.dart:550:49)
#2      Visitor._visitNodeList (package:csslib/visitor.dart:126:19)
#3      Visitor.visitStyleSheet (package:csslib/visitor.dart:134:5)
#4      StyleSheet.visit (package:csslib/src/tree.dart:462:49)
#5      parseCss (package:linkcheck/src/parsers/css.dart:41:11)
#6      checkPage (package:linkcheck/src/worker/worker.dart:161:12)
<asynchronous suspension>
#7      worker.<anonymous closure> (package:linkcheck/src/worker/worker.dart:191:23)
<asynchronous suspension>