Eclipse: Maven project builder generating NullPointerException when building workspace

问题

I'm using STS, with the following version info

Spring Tool Suite 
Version: 3.7.0.RELEASE
Build Id: 201506290652
Platform: Eclipse Mars (4.5.0)

I have imported a number of Maven projects. When Eclipse is automatically building my workspace, this dialog will pop up

Errors occurred during the build.
Errors running builder 'Maven Project Builder' on project 'myproject'.
java.lang.NullPointerException

When I look in my workspace/.metadata/.log file, I see

!ENTRY org.eclipse.core.resources 4 75 2015-07-28 08:40:07.418
!MESSAGE Errors occurred during the build.
!SUBENTRY 1 org.eclipse.m2e.core 4 75 2015-07-28 08:40:07.418
!MESSAGE Errors running builder 'Maven Project Builder' on project 'ebook'.
!STACK 0
java.lang.NullPointerException
        at org.eclipse.wst.common.componentcore.internal.resources.VirtualComponent.setReferences(VirtualComponent.java:501)
        at org.eclipse.m2e.wtp.WebProjectConfiguratorDelegate.setModuleDependencies(WebProjectConfiguratorDelegate.java:282)
        at org.eclipse.m2e.wtp.WTPProjectConfigurator.mavenProjectChanged(WTPProjectConfigurator.java:96)
        at org.eclipse.m2e.core.internal.project.ProjectConfigurationManager.mavenProjectChanged(ProjectConfigurationManager.java:1000)
        at org.eclipse.m2e.core.internal.project.registry.ProjectRegistryManager.notifyProjectChangeListeners(ProjectRegistryManager.java:783)
        at org.eclipse.m2e.core.internal.project.registry.ProjectRegistryManager.applyMutableProjectRegistry(ProjectRegistryManager.java:919)
        at org.eclipse.m2e.core.internal.project.registry.ProjectRegistryManager.refresh(ProjectRegistryManager.java:299)
        at org.eclipse.m2e.core.internal.builder.MavenBuilder$BuildMethod.getProjectFacade(MavenBuilder.java:154)
        at org.eclipse.m2e.core.internal.builder.MavenBuilder$BuildMethod$1.call(MavenBuilder.java:89)
        at org.eclipse.m2e.core.internal.embedder.MavenExecutionContext.executeBare(MavenExecutionContext.java:176)
        at org.eclipse.m2e.core.internal.embedder.MavenExecutionContext.execute(MavenExecutionContext.java:151)
        at org.eclipse.m2e.core.internal.embedder.MavenExecutionContext.execute(MavenExecutionContext.java:99)
        at org.eclipse.m2e.core.internal.builder.MavenBuilder$BuildMethod.execute(MavenBuilder.java:86)
        at org.eclipse.m2e.core.internal.builder.MavenBuilder.build(MavenBuilder.java:200)
        at org.eclipse.core.internal.events.BuildManager$2.run(BuildManager.java:734)
        at org.eclipse.core.runtime.SafeRunner.run(SafeRunner.java:42)
        at org.eclipse.core.internal.events.BuildManager.basicBuild(BuildManager.java:205)
        at org.eclipse.core.internal.events.BuildManager.basicBuild(BuildManager.java:245)
        at org.eclipse.core.internal.events.BuildManager$1.run(BuildManager.java:300)
        at org.eclipse.core.runtime.SafeRunner.run(SafeRunner.java:42)
        at org.eclipse.core.internal.events.BuildManager.basicBuild(BuildManager.java:303)
        at org.eclipse.core.internal.events.BuildManager.basicBuildLoop(BuildManager.java:359)
        at org.eclipse.core.internal.events.BuildManager.build(BuildManager.java:382)
        at org.eclipse.core.internal.events.AutoBuildJob.doBuild(AutoBuildJob.java:144)
        at org.eclipse.core.internal.events.AutoBuildJob.run(AutoBuildJob.java:235)
        at org.eclipse.core.internal.jobs.Worker.run(Worker.java:55)

I have no idea what this means or how to fix it. Help is appreciated, - Dave