staging NullPointerException when moving docker assets via DockerComponentDirector.maybeCopyAssets

Moving a Docker image using the staging API can result in the following NPE:

org.sonatype.nexus.siesta.internal.UnexpectedExceptionMapper - (ID 383168aa-d7fb-4668-b439-d9f7f7c80c7f) Unexpected exception: java.lang.NullPointerException
java.lang.NullPointerException: null
        at org.sonatype.nexus.repository.docker.internal.DockerComponentDirector.maybeCopyAssets(DockerComponentDirector.java:153)
        at org.sonatype.nexus.repository.docker.internal.DockerComponentDirector.beforeMove(DockerComponentDirector.java:108)
        at org.sonatype.nexus.transaction.TransactionalWrapper.proceedWithTransaction(TransactionalWrapper.java:56)
        at org.sonatype.nexus.transaction.TransactionInterceptor.invoke(TransactionInterceptor.java:54)
        at com.sonatype.nexus.staging.internal.StagingComponentManager.move(StagingComponentManager.java:81)
        at com.sonatype.nexus.staging.internal.StagingService.move(StagingService.java:171)
        at org.sonatype.nexus.common.stateguard.MethodInvocationAction.run(MethodInvocationAction.java:39)
        at org.sonatype.nexus.common.stateguard.StateGuard$GuardImpl.run(StateGuard.java:270)
        at org.sonatype.nexus.common.stateguard.GuardedInterceptor.invoke(GuardedInterceptor.java:53)
        at com.sonatype.nexus.staging.internal.StagingResource.move(StagingResource.java:86)
        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.GeneratedMethodAccessor797.invoke(Unknown Source)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:498)
        at org.jboss.resteasy.core.MethodInjectorImpl.invoke(MethodInjectorImpl.java:140)
        at org.jboss.resteasy.core.ResourceMethodInvoker.invokeOnTarget(ResourceMethodInvoker.java:294)
        at org.jboss.resteasy.core.ResourceMethodInvoker.invoke(ResourceMethodInvoker.java:248)
        at org.jboss.resteasy.core.ResourceMethodInvoker.invoke(ResourceMethodInvoker.java:235)
        at org.jboss.resteasy.core.SynchronousDispatcher.invoke(SynchronousDispatcher.java:402)
        at org.jboss.resteasy.core.SynchronousDispatcher.invoke(SynchronousDispatcher.java:209)
        at org.jboss.resteasy.plugins.server.servlet.ServletContainerDispatcher.service(ServletContainerDispatcher.java:227)
        at org.jboss.resteasy.plugins.server.servlet.HttpServletDispatcher.service(HttpServletDispatcher.java:56)
        at org.jboss.resteasy.plugins.server.servlet.HttpServletDispatcher.service(HttpServletDispatcher.java:51)
        at org.sonatype.nexus.siesta.internal.resteasy.ComponentContainerImpl.service(ComponentContainerImpl.java:106)
        at org.sonatype.nexus.siesta.SiestaServlet.service(SiestaServlet.java:137)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:790)

Anecdotal

Triggering a large move of assets as described in NEXUS-19085 may be a way to trigger NEXUS-19181.