NoMethodError in Spree::Checkout#edit when address.state == nil

[mntmn]

I'm testing the checkout with Braintree payment and running into this error as a German test customer. The state field of the address is nil, which is valid in this case (German addresses don't have a state):

`Showing /home/mntmn/.bundle/ruby/2.3.0/solidus_paypal_braintree-898eb2cad543/lib/views/frontend/spree/checkout/payment/_paypal_braintree.html.erb where line #40 raised:

undefined method name' for nil:NilClass

The offending line is:

state: '<%= address.state.name %>',