[MNG-6170] NPE in cases using Multithreaded -T X versions:set

-DnewVersion=1.0-SNAPSHOT
 o In some cases the code will return a null which should
   not be scheduled.

maven-core/src/main/java/org/apache/maven/lifecycle/internal/builder/multithreaded/MultiThreadedBuilder.java

@@ -55,6 +55,7 @@ import org.codehaus.plexus.logging.Logger;
  * @since 3.0
  * @author Kristian Rosenvold
  *         Builds one or more lifecycles for a full module
+ *         NOTE: This class is not part of any public api and can be changed or deleted without prior notice.
  */
 @Component( role = Builder.class, hint = "multithreaded" )
 public class MultiThreadedBuilder
@@ -67,7 +68,6 @@ public class MultiThreadedBuilder
     @Requirement
     private LifecycleModuleBuilder lifecycleModuleBuilder;
 
-
     public MultiThreadedBuilder()
     {
     }
@@ -142,6 +142,10 @@ public class MultiThreadedBuilder
                 {
                     break;
                 }
+
+                // MNG-6170: Only schedule other modules from reactor if we have more modules to build than one. 
+                if ( analyzer.getNumberOfBuilds() > 1 )
+                {
                     final List<MavenProject> newItemsThatCanBeBuilt =
                         analyzer.markAsFinished( projectBuild.getProject() );
                     for ( MavenProject mavenProject : newItemsThatCanBeBuilt )
@@ -153,6 +157,7 @@ public class MultiThreadedBuilder
                         service.submit( cb );
                     }
                 }
+            }
             catch ( InterruptedException e )
             {
                 rootSession.getResult().addException( e );