Upgrade from 1.5.13 to 1.5.19 when updating a merge request now returning 500 error code back to Gitlab.

We are using Jenkins 2.282 running the Gitlab 1.5.13 plugin successfully with the latest version of Gitlab [13.9.3] for at least a year now. A recent Gitlab 1.5.19 plugin was available [seems like it was a jump from 1.5.13 -> 1.5.19 that became available] in Jenkins so we upgraded to the latest version and now our regular Webhooks requests that are triggered during a merge request update are now receiving a 500 error back from Jenkins.

When tailing the Jenkins logs when Gitlab sends the merge request webook to Jenkins, I see the following error in the logs.

```
2021-03-08 19:47:36.692+0000 [id=300] WARNING o.e.j.s.h.ContextHandler$Context#log: Error while serving https://jenkins.myjenkinsite.com/project/Gitlab_Merge_Request_Check
java.lang.IllegalStateException: No credentials found for credentialsId:
at com.dabsquared.gitlabjenkins.connection.GitLabConnection.getApiToken(GitLabConnection.java:155)
at com.dabsquared.gitlabjenkins.connection.GitLabConnection.getClient(GitLabConnection.java:131)
at com.dabsquared.gitlabjenkins.connection.GitLabConnectionConfig.getClient(GitLabConnectionConfig.java:66)
at com.dabsquared.gitlabjenkins.connection.GitLabConnectionProperty.getClient(GitLabConnectionProperty.java:74)
at com.dabsquared.gitlabjenkins.trigger.handler.AbstractWebHookTriggerHandler.setCommitStatusPendingIfNecessary(AbstractWebHookTriggerHandler.java:67)
at com.dabsquared.gitlabjenkins.trigger.handler.AbstractWebHookTriggerHandler.handle(AbstractWebHookTriggerHandler.java:53)
at com.dabsquared.gitlabjenkins.trigger.handler.merge.MergeRequestHookTriggerHandlerImpl.handle(MergeRequestHookTriggerHandlerImpl.java:83)
at com.dabsquared.gitlabjenkins.trigger.handler.merge.MergeRequestHookTriggerHandlerImpl.handle(MergeRequestHookTriggerHandlerImpl.java:41)
at com.dabsquared.gitlabjenkins.GitLabPushTrigger.onPost(GitLabPushTrigger.java:492)
at com.dabsquared.gitlabjenkins.webhook.build.MergeRequestBuildAction$1.performOnPost(MergeRequestBuildAction.java:78)
at com.dabsquared.gitlabjenkins.webhook.build.BuildWebHookAction$TriggerNotifier.run(BuildWebHookAction.java:54)
at hudson.security.ACL.impersonate2(ACL.java:423)
at hudson.security.ACL.impersonate(ACL.java:435)
at com.dabsquared.gitlabjenkins.webhook.build.MergeRequestBuildAction.execute(MergeRequestBuildAction.java:75)
at com.dabsquared.gitlabjenkins.webhook.build.BuildWebHookAction.execute(BuildWebHookAction.java:31)
at com.dabsquared.gitlabjenkins.webhook.GitLabWebHook.getDynamic(GitLabWebHook.java:44)
at java.base/java.lang.invoke.MethodHandle.invokeWithArguments(MethodHandle.java:710)
at org.kohsuke.stapler.Function$MethodFunction.invoke(Function.java:396)
Caused: java.lang.reflect.InvocationTargetException
at org.kohsuke.stapler.Function$MethodFunction.invoke(Function.java:400)
at org.kohsuke.stapler.Function$InstanceFunction.invoke(Function.java:408)
at org.kohsuke.stapler.Function.bindAndInvoke(Function.java:212)
at org.kohsuke.stapler.MetaClass$9.dispatch(MetaClass.java:455)
at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:766)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:898)
at org.kohsuke.stapler.MetaClass$9.dispatch(MetaClass.java:457)
at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:766)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:898)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:694)
at org.kohsuke.stapler.Stapler.service(Stapler.java:240)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:790)
at org.eclipse.jetty.servlet.ServletHolder.handle(ServletHolder.java:791)
at org.eclipse.jetty.servlet.ServletHandler$ChainEnd.doFilter(ServletHandler.java:1626)
at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:154)
at jenkins.telemetry.impl.UserLanguages$AcceptLanguageFilter.doFilter(UserLanguages.java:129)
at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:151)
at jenkins.security.ResourceDomainFilter.doFilter(ResourceDomainFilter.java:76)
at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:151)
at com.cloudbees.jenkins.support.slowrequest.SlowRequestFilter.doFilter(SlowRequestFilter.java:37)
at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:151)
at hudson.plugins.scm_sync_configuration.extensions.ScmSyncConfigurationFilter$1.call(ScmSyncConfigurationFilter.java:49)
at hudson.plugins.scm_sync_configuration.extensions.ScmSyncConfigurationFilter$1.call(ScmSyncConfigurationFilter.java:44)
at hudson.plugins.scm_sync_configuration.ScmSyncConfigurationDataProvider.provideRequestDuring(ScmSyncConfigurationDataProvider.java:106)
at hudson.plugins.scm_sync_configuration.extensions.ScmSyncConfigurationFilter.doFilter(ScmSyncConfigurationFilter.java:44)
at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:151)
at jenkins.metrics.impl.MetricsFilter.doFilter(MetricsFilter.java:125)
at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:151)
at hudson.util.PluginServletFilter.doFilter(PluginServletFilter.java:157)
at org.eclipse.jetty.servlet.FilterHolder.doFilter(FilterHolder.java:193)
at org.eclipse.jetty.servlet.ServletHandler$Chain.doFilter(ServletHandler.java:1601)
at com.dabsquared.gitlabjenkins.webhook.GitLabWebHook$GitlabWebHookCrumbExclusion.process(GitLabWebHook.java:53)
at hudson.security.csrf.CrumbFilter.doFilter(CrumbFilter.java:127)
at org.eclipse.jetty.servlet.FilterHolder.doFilter(FilterHolder.java:193)
at org.eclipse.jetty.servlet.ServletHandler$Chain.doFilter(ServletHandler.java:1601)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:92)
at jenkins.security.AcegiSecurityExceptionFilter.doFilter(AcegiSecurityExceptionFilter.java:52)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:97)
at hudson.security.UnwrapSecurityExceptionFilter.doFilter(UnwrapSecurityExceptionFilter.java:51)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:97)
at org.springframework.security.web.access.ExceptionTranslationFilter.doFilter(ExceptionTranslationFilter.java:119)
at org.springframework.security.web.access.ExceptionTranslationFilter.doFilter(ExceptionTranslationFilter.java:113)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:97)
at org.springframework.security.web.authentication.AnonymousAuthenticationFilter.doFilter(AnonymousAuthenticationFilter.java:105)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:97)
at org.springframework.security.web.authentication.rememberme.RememberMeAuthenticationFilter.doFilter(RememberMeAuthenticationFilter.java:133)
at org.springframework.security.web.authentication.rememberme.RememberMeAuthenticationFilter.doFilter(RememberMeAuthenticationFilter.java:92)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:97)
at org.springframework.security.web.authentication.AbstractAuthenticationProcessingFilter.doFilter(AbstractAuthenticationProcessingFilter.java:218)
at org.springframework.security.web.authentication.AbstractAuthenticationProcessingFilter.doFilter(AbstractAuthenticationProcessingFilter.java:212)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:97)
at jenkins.security.BasicHeaderProcessor.doFilter(BasicHeaderProcessor.java:93)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:97)
at org.springframework.security.web.context.SecurityContextPersistenceFilter.doFilter(SecurityContextPersistenceFilter.java:110)
at org.springframework.security.web.context.SecurityContextPersistenceFilter.doFilter(SecurityContextPersistenceFilter.java:80)
at hudson.security.HttpSessionContextIntegrationFilter2.doFilter(HttpSessionContextIntegrationFilter2.java:62)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:97)
at hudson.security.ChainedServletFilter.doFilter(ChainedServletFilter.java:109)
at hudson.security.HudsonFilter.doFilter(HudsonFilter.java:168)
at org.eclipse.jetty.servlet.FilterHolder.doFilter(FilterHolder.java:193)
at org.eclipse.jetty.servlet.ServletHandler$Chain.doFilter(ServletHandler.java:1601)
at org.kohsuke.stapler.compression.CompressionFilter.doFilter(CompressionFilter.java:51)
at org.eclipse.jetty.servlet.FilterHolder.doFilter(FilterHolder.java:193)
at org.eclipse.jetty.servlet.ServletHandler$Chain.doFilter(ServletHandler.java:1601)
at hudson.util.CharacterEncodingFilter.doFilter(CharacterEncodingFilter.java:82)
at org.eclipse.jetty.servlet.FilterHolder.doFilter(FilterHolder.java:193)
at org.eclipse.jetty.servlet.ServletHandler$Chain.doFilter(ServletHandler.java:1601)
at org.kohsuke.stapler.DiagnosticThreadNameFilter.doFilter(DiagnosticThreadNameFilter.java:30)
at org.eclipse.jetty.servlet.FilterHolder.doFilter(FilterHolder.java:193)
at org.eclipse.jetty.servlet.ServletHandler$Chain.doFilter(ServletHandler.java:1601)
at jenkins.security.SuspiciousRequestFilter.doFilter(SuspiciousRequestFilter.java:36)
at org.eclipse.jetty.servlet.FilterHolder.doFilter(FilterHolder.java:193)
at org.eclipse.jetty.servlet.ServletHandler$Chain.doFilter(ServletHandler.java:1601)
at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:548)
at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:143)
at org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.java:578)
at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:127)
at org.eclipse.jetty.server.handler.ScopedHandler.nextHandle(ScopedHandler.java:235)
at org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandler.java:1624)
at org.eclipse.jetty.server.handler.ScopedHandler.nextHandle(ScopedHandler.java:233)
at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1435)
at org.eclipse.jetty.server.handler.ScopedHandler.nextScope(ScopedHandler.java:188)
at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:501)
at org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandler.java:1594)
at org.eclipse.jetty.server.handler.ScopedHandler.nextScope(ScopedHandler.java:186)
at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1350)
at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:141)
at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:127)
at org.eclipse.jetty.server.Server.handle(Server.java:516)
at org.eclipse.jetty.server.HttpChannel.lambda$handle$1(HttpChannel.java:388)
at org.eclipse.jetty.server.HttpChannel.dispatch(HttpChannel.java:633)
at org.eclipse.jetty.server.HttpChannel.handle(HttpChannel.java:380)
at org.eclipse.jetty.server.HttpConnection.onFillable(HttpConnection.java:279)
at org.eclipse.jetty.io.AbstractConnection$ReadCallback.succeeded(AbstractConnection.java:311)
at org.eclipse.jetty.io.FillInterest.fillable(FillInterest.java:105)
at org.eclipse.jetty.io.ChannelEndPoint$1.run(ChannelEndPoint.java:104)
at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.runTask(EatWhatYouKill.java:336)
at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.doProduce(EatWhatYouKill.java:313)
at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.tryProduce(EatWhatYouKill.java:171)
at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.run(EatWhatYouKill.java:129)
at org.eclipse.jetty.util.thread.ReservedThreadExecutor$ReservedThread.run(ReservedThreadExecutor.java:383)
at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:882)
at org.eclipse.jetty.util.thread.QueuedThreadPool$Runner.run(QueuedThreadPool.java:1036)
at java.base/java.lang.Thread.run(Thread.java:834)
```

However, When I downgrade the Gitlab plugin back to 1.5.13, it all works as normal.