Caused by: net.rcarz.jiraclient.JiraException: Field 'priority' does not exist or read-only

[123dev]

We're leaving the optional Priority field blank as our new workflow does not allow priority setting during the ticket creation.
Even though we left it blank and it's optional, the plugin is still trying to set the value and is failing to create the ticket with the following exception.

2018-06-15_15:48:14.64614 org.graylog2.plugin.alarms.callbacks.AlarmCallbackException: Failed creating new issue
2018-06-15_15:48:14.66703       at com.bidorbuy.graylog.alarmcallbacks.jira.JiraIssueClient.createJIRAIssue(JiraIssueClient.java:213) ~[graylog-plugin-jira-1.0.8.jar:?]
2018-06-15_15:48:14.66725       at com.bidorbuy.graylog.alarmcallbacks.jira.JiraIssueClient.trigger(JiraIssueClient.java:80) [graylog-plugin-jira-1.0.8.jar:?]
2018-06-15_15:48:14.66792       at com.bidorbuy.graylog.alarmcallbacks.jira.JiraAlarmCallback.call(JiraAlarmCallback.java:153) [graylog-plugin-jira-1.0.8.jar:?]
2018-06-15_15:48:14.67057       at com.bidorbuy.graylog.alarmcallbacks.jira.JiraAlarmCallback.call(JiraAlarmCallback.java:128) [graylog-plugin-jira-1.0.8.jar:?]
2018-06-15_15:48:14.67100       at org.graylog2.rest.resources.alarmcallbacks.AlarmCallbacksResource.test(AlarmCallbacksResource.java:150) [graylog.jar:?]
2018-06-15_15:48:14.67209       at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_161]
2018-06-15_15:48:14.67259       at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[?:1.8.0_161]
2018-06-15_15:48:14.67371       at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:1.8.0_161]
2018-06-15_15:48:14.67416       at java.lang.reflect.Method.invoke(Method.java:498) ~[?:1.8.0_161]
2018-06-15_15:48:14.67571       at org.glassfish.jersey.server.model.internal.ResourceMethodInvocationHandlerFactory$1.invoke(ResourceMethodInvocationHandlerFactory.java:81) [graylog.jar:?]
2018-06-15_15:48:14.68066       at org.glassfish.jersey.server.model.internal.AbstractJavaResourceMethodDispatcher$1.run(AbstractJavaResourceMethodDispatcher.java:144) [graylog.jar:?]
2018-06-15_15:48:14.68068       at org.glassfish.jersey.server.model.internal.AbstractJavaResourceMethodDispatcher.invoke(AbstractJavaResourceMethodDispatcher.java:161) [graylog.jar:?]
2018-06-15_15:48:14.68068       at org.glassfish.jersey.server.model.internal.JavaResourceMethodDispatcherProvider$ResponseOutInvoker.doDispatch(JavaResourceMethodDispatcherProvider.java:160) [graylog.jar:?]
2018-06-15_15:48:14.68069       at org.glassfish.jersey.server.model.internal.AbstractJavaResourceMethodDispatcher.dispatch(AbstractJavaResourceMethodDispatcher.java:99) [graylog.jar:?]
2018-06-15_15:48:14.68069       at org.glassfish.jersey.server.model.ResourceMethodInvoker.invoke(ResourceMethodInvoker.java:389) [graylog.jar:?]
2018-06-15_15:48:14.68070       at org.glassfish.jersey.server.model.ResourceMethodInvoker.apply(ResourceMethodInvoker.java:347) [graylog.jar:?]
2018-06-15_15:48:14.68070       at org.glassfish.jersey.server.model.ResourceMethodInvoker.apply(ResourceMethodInvoker.java:102) [graylog.jar:?]
2018-06-15_15:48:14.68070       at org.glassfish.jersey.server.ServerRuntime$2.run(ServerRuntime.java:326) [graylog.jar:?]
2018-06-15_15:48:14.68072       at org.glassfish.jersey.internal.Errors$1.call(Errors.java:271) [graylog.jar:?]
2018-06-15_15:48:14.68072       at org.glassfish.jersey.internal.Errors$1.call(Errors.java:267) [graylog.jar:?]
2018-06-15_15:48:14.68927       at org.glassfish.jersey.internal.Errors.process(Errors.java:315) [graylog.jar:?]
2018-06-15_15:48:14.68982       at org.glassfish.jersey.internal.Errors.process(Errors.java:297) [graylog.jar:?]
2018-06-15_15:48:14.69059       at org.glassfish.jersey.internal.Errors.process(Errors.java:267) [graylog.jar:?]
2018-06-15_15:48:14.69790       at org.glassfish.jersey.process.internal.RequestScope.runInScope(RequestScope.java:317) [graylog.jar:?]
2018-06-15_15:48:14.69880       at org.glassfish.jersey.server.ServerRuntime.process(ServerRuntime.java:305) [graylog.jar:?]
2018-06-15_15:48:14.70103       at org.glassfish.jersey.server.ApplicationHandler.handle(ApplicationHandler.java:1154) [graylog.jar:?]
2018-06-15_15:48:14.70171       at org.glassfish.jersey.grizzly2.httpserver.GrizzlyHttpContainer.service(GrizzlyHttpContainer.java:384) [graylog.jar:?]
2018-06-15_15:48:14.70304       at org.glassfish.grizzly.http.server.HttpHandler$1.run(HttpHandler.java:224) [graylog.jar:?]
2018-06-15_15:48:14.70370       at com.codahale.metrics.InstrumentedExecutorService$InstrumentedRunnable.run(InstrumentedExecutorService.java:176) [graylog.jar:?]
2018-06-15_15:48:14.70514       at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) [?:1.8.0_161]
2018-06-15_15:48:14.70570       at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) [?:1.8.0_161]
2018-06-15_15:48:14.70686       at java.lang.Thread.run(Thread.java:748) [?:1.8.0_161]
2018-06-15_15:48:14.70756 Caused by: net.rcarz.jiraclient.JiraException: Field 'priority' does not exist or read-only
2018-06-15_15:48:14.70901       at net.rcarz.jiraclient.Field.getFieldMetadata(Field.java:432) ~[?:?]
2018-06-15_15:48:14.70965       at net.rcarz.jiraclient.Field.toJson(Field.java:535) ~[?:?]
2018-06-15_15:48:14.71111       at net.rcarz.jiraclient.Issue$FluentCreate.executeCreate(Issue.java:104) ~[?:?]
2018-06-15_15:48:14.71188       at net.rcarz.jiraclient.Issue$FluentCreate.execute(Issue.java:59) ~[?:?]
2018-06-15_15:48:14.71267       at com.bidorbuy.graylog.alarmcallbacks.jira.JiraIssueClient.createJIRAIssue(JiraIssueClient.java:208) ~[?:?]
2018-06-15_15:48:14.71318       ... 31 more

On the same topic, we are planning to add a required field 'Severity' to ticket creation.
How can we set this value with the plugin, the only option we see is: JIRA/Graylog field mapping
but the value should be fixed, and not dynamically set by Graylog field.
Example: Severity: Critical

Thanks in advance.