I am getting a similar error after upgrading to Elasticsearch 7.0.0 (was working in 6.3).
I am getting
[2019-04-14T09:15:36,933][ERROR][logstash.outputs.elasticsearch] Failed to install template. {:message=>"undefined method `update' for nil:NilClass", :class=>"NoMethodError", :backtrace=>["/usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/logstash-output-elasticsearch-10.0.1-java/lib/logstash/outputs/elasticsearch/template_manager.rb:39:in `add_ilm_settings_to_template'", "/usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/logstash-output-elasticsearch-10.0.1-java/lib/logstash/outputs/elasticsearch/template_manager.rb:14:in `install_template'", "/usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/logstash-output-elasticsearch-10.0.1-java/lib/logstash/outputs/elasticsearch/common.rb:129:in `install_template'", "/usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/logstash-output-elasticsearch-10.0.1-java/lib/logstash/outputs/elasticsearch/common.rb:50:in `block in setup_after_successful_connection'"]}