After upgrade to Maven Integration 3.18 (from 3.16) our Maven 2 projects fail to build with

ERROR: Processing failed due to a bug in the code. Please report this to the issue tracker (https://jenkins.io/redirect/report-an-issue).
java.lang.NullPointerException
at hudson.Util.getDigestOf(Util.java:637)
at hudson.maven.MavenProcessFactory.getMavenAgentClassPath(MavenProcessFactory.java:72)
at hudson.maven.AbstractMavenProcessFactory.buildMavenAgentCmdLine(AbstractMavenProcessFactory.java:415)
at hudson.maven.AbstractMavenProcessFactory.newProcess(AbstractMavenProcessFactory.java:278)
at hudson.maven.ProcessCache.get(ProcessCache.java:236)
at hudson.maven.MavenModuleSetBuild$MavenModuleSetBuildExecution.doRun(MavenModuleSetBuild.java:802)
at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:522)
at hudson.model.Run.execute(Run.java:1896)
at hudson.maven.MavenModuleSetBuild.run(MavenModuleSetBuild.java:543)
at hudson.model.ResourceController.execute(ResourceController.java:101)
at hudson.model.Executor.run(Executor.java:442)

The Maven 3 projects still build fine. After reverting to Maven Integration 3.16 the Maven 2 projects can be build again.

There is a difference in the console output:

Maven Integration 3.18:

Parsing POMs
Modules changed, recalculating dependency graph
Established TCP socket on 42899
maven-agent.jar already up to date

java.lang.NullPointerException...

vs.

Maven Integration 3.16

Parsing POMs
Modules changed, recalculating dependency graph
Established TCP socket on 34158
maven-agent.jar already up to date
classworlds.jar already up to date
maven-interceptor.jar already up to date
[maven2-project] $ /opt/buildsystem/java/jdk-8u201-ojdkbuild-linux-x64/bin/java -Xms256m -Xmx2048m -cp /opt/buildsystem/maven-agent.jar:/opt/buildsystem/classworlds.jar hudson.maven.agent.Main /opt/buildsystem/maven/apache-maven-2.0.11 /opt/buildsystem/agent.jar /opt/buildsystem/maven-interceptor.jar 34158
<===[JENKINS REMOTING CAPACITY]===>  channel started
Executing Maven: -B -f /opt/buildsystem/workspace/maven2-project/parent/pom.xml clean deploy