What code were you trying to parse?

var a: function(b): c;

What actually happened?

Cannot set property 'typeAnnotation' of null

I haven't debugged this at all, but I assume it's because the variable's type declaration is of kind JSDocFunctionType, which we do not have any handling for.

https://astexplorer.net/#/gist/a1a5bcad94191c0d730eb920ced2e24b/f10df5321aaf8ceef561d14ce57a0ab59eee0ac5