Description of problem:
-----------------------
The UI blows up with this error while assigning a tag to an infra host.

undefined method `push' for nil:NilClass [host/tag_edit_form_field_changed]

Here's a snippet from production.log:

[----] I, [2018-11-05T16:13:42.421098 #15255:b1510c]  INFO -- : Started POST "/host/tag_edit_form_field_changed/1?tag_cat=1" for 127.0.0.1 at 2018-11-05 16:13:42 -0500
[----] I, [2018-11-05T16:13:42.424348 #15255:b1510c]  INFO -- : Processing by HostController#tag_edit_form_field_changed as JS
[----] I, [2018-11-05T16:13:42.424438 #15255:b1510c]  INFO -- :   Parameters: {"tag_cat"=>"1", "id"=>"1"}
[----] F, [2018-11-05T16:13:42.441135 #15255:b1510c] FATAL -- : Error caught: [ActiveRecord::RecordNotFound] Couldn't find Classification with 'id'=
/opt/rh/cfme-gemset/gems/activerecord-5.0.7/lib/active_record/core.rb:173:in `find'
/opt/rh/cfme-gemset/bundler/gems/cfme-ui-classic-c40152cd3d6e/app/controllers/application_controller/tags.rb:255:in `tag_edit_build_entries_pulldown'
/opt/rh/cfme-gemset/bundler/gems/cfme-ui-classic-c40152cd3d6e/app/controllers/application_controller/tags.rb:71:in `tag_set_vars_from_params'
/opt/rh/cfme-gemset/bundler/gems/cfme-ui-classic-c40152cd3d6e/app/controllers/application_controller/tags.rb:48:in `tag_edit_form_field_changed'
/opt/rh/cfme-gemset/gems/actionpack-5.0.7/lib/action_controller/metal/basic_implicit_render.rb:4:in `send_action'
/opt/rh/cfme-gemset/gems/actionpack-5.0.7/lib/abstract_controller/base.rb:188:in `process_action'
/opt/rh/cfme-gemset/gems/actionpack-5.0.7/lib/action_controller/metal/rendering.rb:30:in `process_action'
/opt/rh/cfme-gemset/gems/actionpack-5.0.7/lib/abstract_controller/callbacks.rb:20:in `block in process_action'
/opt/rh/cfme-gemset/gems/activesupport-5.0.7/lib/active_support/callbacks.rb:126:in `call'
/opt/rh/cfme-gemset/gems/activesupport-5.0.7/lib/active_support/callbacks.rb:506:in `block (2 levels) in compile'
/opt/rh/cfme-gemset/gems/activesupport-5.0.7/lib/active_support/callbacks.rb:455:in `call'
/opt/rh/cfme-gemset/gems/activesupport-5.0.7/lib/active_support/callbacks.rb:101:in `__run_callbacks__'
/opt/rh/cfme-gemset/gems/activesupport-5.0.7/lib/active_support/callbacks.rb:750:in `_run_process_action_callbacks'
/opt/rh/cfme-gemset/gems/activesupport-5.0.7/lib/active_support/callbacks.rb:90:in `run_callbacks'
/opt/rh/cfme-gemset/gems/actionpack-5.0.7/lib/abstract_controller/callbacks.rb:19:in `process_action'
/opt/rh/cfme-gemset/gems/actionpack-5.0.7/lib/action_controller/metal/rescue.rb:20:in `process_action'
/opt/rh/cfme-gemset/gems/actionpack-5.0.7/lib/action_controller/metal/instrumentation.rb:32:in `block in process_action'
/opt/rh/cfme-gemset/gems/activesupport-5.0.7/lib/active_support/notifications.rb:164:in `block in instrument'
/opt/rh/cfme-gemset/gems/activesupport-5.0.7/lib/active_support/notifications/instrumenter.rb:21:in `instrument'
/opt/rh/cfme-gemset/gems/activesupport-5.0.7/lib/active_support/notifications.rb:164:in `instrument'
/opt/rh/cfme-gemset/gems/actionpack-5.0.7/lib/action_controller/metal/instrumentation.rb:30:in `process_action'
/opt/rh/cfme-gemset/gems/actionpack-5.0.7/lib/action_controller/metal/params_wrapper.rb:248:in `process_action'
/opt/rh/cfme-gemset/gems/activerecord-5.0.7/lib/active_record/railties/controller_runtime.rb:18:in `process_action'
/opt/rh/cfme-gemset/gems/actionpack-5.0.7/lib/abstract_controller/base.rb:126:in `process'
/opt/rh/cfme-gemset/gems/actionview-5.0.7/lib/action_view/rendering.rb:30:in `process'
/opt/rh/cfme-gemset/gems/actionpack-5.0.7/lib/action_controller/metal.rb:190:in `dispatch'
/opt/rh/cfme-gemset/gems/actionpack-5.0.7/lib/action_controller/metal.rb:262:in `dispatch'
/opt/rh/cfme-gemset/gems/actionpack-5.0.7/lib/action_dispatch/routing/route_set.rb:50:in `dispatch'
/opt/rh/cfme-gemset/gems/actionpack-5.0.7/lib/action_dispatch/routing/route_set.rb:32:in `serve' 
/opt/rh/cfme-gemset/gems/actionpack-5.0.7/lib/action_dispatch/journey/router.rb:39:in `block in serve'
/opt/rh/cfme-gemset/gems/actionpack-5.0.7/lib/action_dispatch/journey/router.rb:26:in `each'
/opt/rh/cfme-gemset/gems/actionpack-5.0.7/lib/action_dispatch/journey/router.rb:26:in `serve' 
/opt/rh/cfme-gemset/gems/actionpack-5.0.7/lib/action_dispatch/routing/route_set.rb:727:in `call'
/opt/rh/cfme-gemset/bundler/gems/cfme-graphql-5751b531db81/lib/manageiq/graphql/rest_api_proxy.rb:18:in `call'
/opt/rh/cfme-gemset/gems/secure_headers-3.0.3/lib/secure_headers/middleware.rb:10:in `call'


Version-Release number of selected component (if applicable):
-------------------------------------------------------------
5.10.0.22


How reproducible:
-----------------
Always


Steps to Reproduce:
-------------------
1.Manage an infra provider such as RHV 
2.Navigate to the summary page of a host.
3.Click 'Policy', select 'Edit tags' to assign a tag to a host.


Actual results:
--------------
UI blows up


Expected results:
-----------------
No UI errors


Additional info:
----------------