Updated to the new Warnings Next Generation plugin 8.4.2 version this morning and rebuilt my warnings job ( using GNU C Compiler gcc )
( clang tidy job also fails with NullPointerException )

java.lang.NullPointerException
at edu.hm.hafner.analysis.Report.addAll(Report.java:174)
at edu.hm.hafner.analysis.Report.copyIssuesAndProperties(Report.java:660)
at edu.hm.hafner.analysis.Report.addAll(Report.java:194)
at io.jenkins.plugins.analysis.core.model.AnalysisResult.getIssues(AnalysisResult.java:413)
at java.util.Optional.map(Optional.java:215)
at io.jenkins.plugins.analysis.core.model.AnalysisHistory.getIssues(AnalysisHistory.java:142)
at io.jenkins.plugins.analysis.core.model.DeltaReport.<init>(DeltaReport.java:48)
at io.jenkins.plugins.analysis.core.steps.IssuesPublisher.attachAction(IssuesPublisher.java:110)
at io.jenkins.plugins.analysis.core.steps.IssuesRecorder.publishResult(IssuesRecorder.java:721)
at io.jenkins.plugins.analysis.core.steps.IssuesRecorder.record(IssuesRecorder.java:651)
at io.jenkins.plugins.analysis.core.steps.IssuesRecorder.perform(IssuesRecorder.java:617)
at io.jenkins.plugins.analysis.core.steps.IssuesRecorder.perform(IssuesRecorder.java:595)
at hudson.tasks.BuildStepMonitor$1.perform(BuildStepMonitor.java:20)
at hudson.model.AbstractBuild$AbstractBuildExecution.perform(AbstractBuild.java:741)
at hudson.model.AbstractBuild$AbstractBuildExecution.performAllBuildSteps(AbstractBuild.java:690)
at hudson.model.Build$BuildExecution.post2(Build.java:186)
at hudson.model.AbstractBuild$AbstractBuildExecution.post(AbstractBuild.java:635)
at hudson.model.Run.execute(Run.java:1919)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:43)
at hudson.model.ResourceController.execute(ResourceController.java:97)
at hudson.model.Executor.run(Executor.java:428)

Previous builds not visible - Jenkins Oops message.
Reverted to 8.4.1 and my old results have No Issues even though the graph shows the correct number of warnings.