Upgraded from slack plugin 1.8.1 to 2.1 got this error.

hudson.util.HudsonFailedToLoad: java.lang.AssertionError: class jenkins.plugins.slack.SlackNotifier$SlackJobProperty is missing its descriptor
at hudson.WebAppMain$3.run(WebAppMain.java:245)
Caused by: java.lang.AssertionError: class jenkins.plugins.slack.SlackNotifier$SlackJobProperty is missing its descriptor
at jenkins.model.Jenkins.getDescriptorOrDie(Jenkins.java:1533)
at hudson.model.JobProperty.getDescriptor(JobProperty.java:105)
at hudson.model.JobProperty.getDescriptor(JobProperty.java:79)
at hudson.model.Descriptor.toMap(Descriptor.java:989)
at hudson.model.Job.getProperties(Job.java:554)
at org.jenkinsci.plugins.envinject.migration.EnvInjectMigrationListener.onLoaded(EnvInjectMigrationListener.java:47)
at jenkins.model.Jenkins.<init>(Jenkins.java:995)
at hudson.model.Hudson.<init>(Hudson.java:85)
at hudson.model.Hudson.<init>(Hudson.java:81)
at hudson.WebAppMain$3.run(WebAppMain.java:231)