core: avoid crash case with nil [GH-6730]

CHANGELOG.md

@@ -6,6 +6,8 @@ BUG FIXES:
   - core: Fix exception on installing VirtualBox [GH-6713]
   - core: Do not convert standalone drive letters such as "D:" to
       UNC paths [GH-6598]
+  - core: Fix a crash in parsing the config in some cases with network
+      configurations [GH-6730]
   - commands/up: Smarter logic about what provider to install, avoiding
       situations where VirtualBox was installed over the correct provider [GH-6731]
   - guests/debian: Fix Docker install [GH-6722]

plugins/kernel_v2/config/vm.rb

@@ -698,7 +698,7 @@ module VagrantPlugins
               end
             end
 
-            if options[:ip] && options[:ip].end_with?(".1") && options[:type].to_sym != :dhcp
+            if options[:ip] && options[:ip].end_with?(".1") && (options[:type] || "").to_sym != :dhcp
               machine.ui.warn(I18n.t(
                 "vagrant.config.vm.network_ip_ends_in_one"))
             end