Hi,
could it be a recent change (last 1-3 days), possibly:
https://github.com/discourse/discourse/commit/3f8fa4ad4e6e27070b213d52618fd66054ab71fb
created an issue when posting new topics as non admin users:
NoMethodError (undefined method `posts' for nil:NilClass)
/var/www/discourse/lib/validators/post_validator.rb:149:in `force_edit_last_validator'
/var/www/discourse/lib/validators/post_validator.rb:149:in `force_edit_last_validator'
/var/www/discourse/lib/validators/post_validator.rb:20:in `validate'
/var/www/discourse/lib/post_creator.rb:156:in `valid?'
/var/www/discourse/lib/post_creator.rb:164:in `create'
/var/www/discourse/lib/new_post_manager.rb:204:in `perform_create_post'
/var/www/discourse/lib/new_post_manager.rb:178:in `perform'
/var/www/discourse/app/controllers/posts_controller.rb:185:in `create'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/actionpack-5.2.0/lib/action_controller/metal/basic_implicit_render.rb:6:in `send_action'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/actionpack-5.2.0/lib/abstract_controller/base.rb:194:in `process_action'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/actionpack-5.2.0/lib/action_controller/metal/rendering.rb:30:in `process_action'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/actionpack-5.2.0/lib/abstract_controller/callbacks.rb:42:in `block in process_action'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/activesupport-5.2.0/lib/active_support/callbacks.rb:132:in `run_callbacks'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/actionpack-5.2.0/lib/abstract_controller/callbacks.rb:41:in `process_action'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/actionpack-5.2.0/lib/action_controller/metal/rescue.rb:22:in `process_action'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/actionpack-5.2.0/lib/action_controller/metal/instrumentation.rb:34:in `block in process_action'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/activesupport-5.2.0/lib/active_support/notifications.rb:168:in `block in instrument'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/activesupport-5.2.0/lib/active_support/notifications/instrumenter.rb:23:in `instrument'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/activesupport-5.2.0/lib/active_support/notifications.rb:168:in `instrument'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/actionpack-5.2.0/lib/action_controller/metal/instrumentation.rb:32:in `process_action'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/actionpack-5.2.0/lib/action_controller/metal/params_wrapper.rb:256:in `process_action'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/activerecord-5.2.0/lib/active_record/railties/controller_runtime.rb:24:in `process_action'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/actionpack-5.2.0/lib/abstract_controller/base.rb:134:in `process'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/actionview-5.2.0/lib/action_view/rendering.rb:32:in `process'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/rack-mini-profiler-1.0.0/lib/mini_profiler/profiling_methods.rb:78:in `block in profile_method'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/actionpack-5.2.0/lib/action_controller/metal.rb:191:in `dispatch'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/actionpack-5.2.0/lib/action_controller/metal.rb:252:in `dispatch'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/actionpack-5.2.0/lib/action_dispatch/routing/route_set.rb:52:in `dispatch'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/actionpack-5.2.0/lib/action_dispatch/routing/route_set.rb:34:in `serve'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/actionpack-5.2.0/lib/action_dispatch/journey/router.rb:52:in `block in serve'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/actionpack-5.2.0/lib/action_dispatch/journey/router.rb:35:in `each'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/actionpack-5.2.0/lib/action_dispatch/journey/router.rb:35:in `serve'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/actionpack-5.2.0/lib/action_dispatch/routing/route_set.rb:840:in `call'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/rack-protection-2.0.3/lib/rack/protection/frame_options.rb:31:in `call'
/var/www/discourse/lib/middleware/omniauth_bypass_middleware.rb:26:in `call'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/rack-2.0.6/lib/rack/tempfile_reaper.rb:15:in `call'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/rack-2.0.6/lib/rack/conditional_get.rb:38:in `call'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/rack-2.0.6/lib/rack/head.rb:12:in `call'
/var/www/discourse/lib/content_security_policy/middleware.rb:12:in `call'
/var/www/discourse/lib/middleware/anonymous_cache.rb:216:in `call'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/rack-2.0.6/lib/rack/session/abstract/id.rb:232:in `context'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/rack-2.0.6/lib/rack/session/abstract/id.rb:226:in `call'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/actionpack-5.2.0/lib/action_dispatch/middleware/cookies.rb:670:in `call'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/actionpack-5.2.0/lib/action_dispatch/middleware/callbacks.rb:28:in `block in call'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/activesupport-5.2.0/lib/active_support/callbacks.rb:98:in `run_callbacks'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/actionpack-5.2.0/lib/action_dispatch/middleware/callbacks.rb:26:in `call'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/actionpack-5.2.0/lib/action_dispatch/middleware/debug_exceptions.rb:61:in `call'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/actionpack-5.2.0/lib/action_dispatch/middleware/show_exceptions.rb:33:in `call'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/logster-1.3.1/lib/logster/middleware/reporter.rb:31:in `call'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/railties-5.2.0/lib/rails/rack/logger.rb:38:in `call_app'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/railties-5.2.0/lib/rails/rack/logger.rb:28:in `call'
/var/www/discourse/config/initializers/100-quiet_logger.rb:16:in `call'
/var/www/discourse/config/initializers/100-silence_logger.rb:29:in `call'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/actionpack-5.2.0/lib/action_dispatch/middleware/remote_ip.rb:81:in `call'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/actionpack-5.2.0/lib/action_dispatch/middleware/request_id.rb:27:in `call'
/var/www/discourse/lib/middleware/enforce_hostname.rb:17:in `call'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/rack-2.0.6/lib/rack/method_override.rb:22:in `call'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/actionpack-5.2.0/lib/action_dispatch/middleware/executor.rb:14:in `call'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/rack-2.0.6/lib/rack/sendfile.rb:111:in `call'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/rack-mini-profiler-1.0.0/lib/mini_profiler/profiler.rb:174:in `call'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/message_bus-2.2.0.pre.1/lib/message_bus/rack/middleware.rb:57:in `call'
/var/www/discourse/lib/middleware/request_tracker.rb:180:in `call'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/railties-5.2.0/lib/rails/engine.rb:524:in `call'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/railties-5.2.0/lib/rails/railtie.rb:190:in `public_send'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/railties-5.2.0/lib/rails/railtie.rb:190:in `method_missing'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/rack-2.0.6/lib/rack/urlmap.rb:68:in `block in call'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/rack-2.0.6/lib/rack/urlmap.rb:53:in `each'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/rack-2.0.6/lib/rack/urlmap.rb:53:in `call'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/unicorn-5.4.0/lib/unicorn/http_server.rb:606:in `process_client'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/unicorn-5.4.0/lib/unicorn/http_server.rb:701:in `worker_loop'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/unicorn-5.4.0/lib/unicorn/http_server.rb:549:in `spawn_missing_workers'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/unicorn-5.4.0/lib/unicorn/http_server.rb:142:in `start'
/var/www/discourse/vendor/bundle/ruby/2.5.0/gems/unicorn-5.4.0/bin/unicorn:126:in `<top (required)>'
/var/www/discourse/vendor/bundle/ruby/2.5.0/bin/unicorn:23:in `load'
/var/www/discourse/vendor/bundle/ruby/2.5.0/bin/unicorn:23:in `<main>'