Save of LDAP user and group settings fails with error.

Set up an LDAP server using "simple authentication" authentication method. Also configure "user and group" settings.  Save.

Now go to the "user and group" settings, and make a change to them, and attempt to save.  This will fail with UI error shown as "connection.systemUsername".

The log shows;

 2020-05-12 11:21:10,385-0500 ERROR [qtp743319359-197] admin org.sonatype.nexus.extdirect.internal.ExtDirectExceptionHandler - Failed to invoke action method: ldap_LdapServer.update, java-method: org.sonatype.nexus.ldap.internal.ui.LdapServerComponent.update
java.lang.IllegalArgumentException: connection.systemUsername
at com.google.common.base.Preconditions.checkArgument(Preconditions.java:135)
at org.sonatype.nexus.ldap.persist.internal.Validator.validate(Validator.java:53)
at org.sonatype.nexus.ldap.persist.internal.Validator.validate(Validator.java:39)
at org.sonatype.nexus.ldap.persist.internal.DefaultLdapConfigurationManager.updateLdapServerConfiguration(DefaultLdapConfigurationManager.java:167)
at org.sonatype.nexus.ldap.persist.LdapConfigurationManager$updateLdapServerConfiguration$2.call(Unknown Source)
at org.sonatype.nexus.ldap.internal.ui.LdapServerComponent.update(LdapServerComponent.groovy:137)
at com.palominolabs.metrics.guice.ExceptionMeteredInterceptor.invoke(ExceptionMeteredInterceptor.java:23)
at com.palominolabs.metrics.guice.TimedInterceptor.invoke(TimedInterceptor.java:26)
at org.sonatype.nexus.validation.internal.ValidationInterceptor.invoke(ValidationInterceptor.java:53)
at org.apache.shiro.guice.aop.AopAllianceMethodInvocationAdapter.proceed(AopAllianceMethodInvocationAdapter.java:49)
at org.apache.shiro.authz.aop.AuthorizingAnnotationMethodInterceptor.invoke(AuthorizingAnnotationMethodInterceptor.java:68)
at org.apache.shiro.guice.aop.AopAllianceMethodInterceptorAdapter.invoke(AopAllianceMethodInterceptorAdapter.java:36)
at org.apache.shiro.guice.aop.AopAllianceMethodInvocationAdapter.proceed(AopAllianceMethodInvocationAdapter.java:49)
at org.apache.shiro.authz.aop.AuthorizingAnnotationMethodInterceptor.invoke(AuthorizingAnnotationMethodInterceptor.java:68)
at org.apache.shiro.guice.aop.AopAllianceMethodInterceptorAdapter.invoke(AopAllianceMethodInterceptorAdapter.java:36)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)

This is a regression introduced by the changes made for NEXUS-23556.

Workaround: Enter the system user password on the connection screen, do not save, go to "user and group" settings screen make change, and save.

Make a change to the "user and group" settings screen, save.