core: fix crash case with provisioners [GH-4281]

CHANGELOG.md

@@ -24,6 +24,7 @@ BUG FIXES:
   - core: Windows is not detected as NixOS in some cases. [GH-4302]
   - core: Fix encoding issues with Windows. There are still some outlying
       but this fixes a few. [GH-4159]
+  - core: Fix crash case when destroying with an invalid provisioner. [GH-4281]
   - commands/package: base package won't crash with exception [GH-4017]
   - communicators/winrm: Support `mkdir` [GH-4271]
   - guests/centos: Fix issues when NFS client is installed by restarting

lib/vagrant/action/builtin/mixin_provisioners.rb

@@ -16,6 +16,10 @@ module Vagrant
           @_provisioner_instances = env[:machine].config.vm.provisioners.map do |provisioner|
             # Instantiate the provisioner
             klass  = Vagrant.plugin("2").manager.provisioners[provisioner.name]
+
+            # This can happen in the case the configuration isn't validated.
+            next nil if !klass
+
             result = klass.new(env[:machine], provisioner.config)
 
             # Store in the type map so that --provision-with works properly
@@ -30,7 +34,7 @@ module Vagrant
             [result, options]
           end
 
-          return @_provisioner_instances
+          return @_provisioner_instances.compact
         end
 
         # This will return a mapping of a provisioner instance to its