undefined method `arguments' for nil:NilClass in Lint/AmbiguousOperator

[mwean]

I ran into this error that I think is caused by the safe navigation operator.

---

Expected behavior

No error when running Lint/AmbiguousOperator on a file containing something like foo&.* -1

Actual behavior

undefined method `arguments' for nil:NilClass
gems/rubocop-0.90.0/lib/rubocop/cop/util.rb:37:in `add_parentheses'
gems/rubocop-0.90.0/lib/rubocop/cop/lint/ambiguous_operator.rb:51:in `block (2 levels) in on_new_investigation'
...

Steps to reproduce the problem

Run rubocop --only Lint/AmbiguousOperator -d --force-default-config on a file containing foo&.* -1

RuboCop version

$ [bundle exec] rubocop -V
0.90.0 (using Parser 2.7.1.4, rubocop-ast 0.4.0, running on ruby 2.5.7 x86_64-darwin18)