Fix mojohaus#59: prevent NPE in Eclipse/m2e

src/main/java/org/codehaus/mojo/flatten/FlattenMojo.java

@@ -72,6 +72,7 @@
 import java.io.Writer;
 import java.util.ArrayList;
 import java.util.Collection;
+import java.util.Collections;
 import java.util.List;
 import java.util.Properties;
 
@@ -698,7 +699,7 @@ private ModelBuildingRequest createModelBuildingRequest( File pomFile )
     {
 
         FlattenModelResolver resolver = new FlattenModelResolver( this.localRepository, this.artifactFactory,
-            this.dependencyResolver, this.session.getProjectBuildingRequest(), this.session.getAllProjects() );
+            this.dependencyResolver, this.session.getProjectBuildingRequest(), getReactorModelsFromSession() );
         Properties userProperties = this.session.getUserProperties();
         List<String> activeProfiles = this.session.getRequest().getActiveProfiles();
 
@@ -709,6 +710,21 @@ private ModelBuildingRequest createModelBuildingRequest( File pomFile )
         return buildingRequest;
     }
 
+    private List<MavenProject> getReactorModelsFromSession()
+    {
+        // robust approach for 'special' environments like m2e (Eclipse plugin) which don't provide allProjects
+        List<MavenProject> models = this.session.getAllProjects();
+        if ( models == null )
+        {
+            models = this.session.getProjects();
+        }
+        if ( models == null )
+        {
+            models = Collections.emptyList();
+        }
+        return models;
+    }
+
     /**
      * Creates the effective POM for the given <code>pomFile</code> trying its best to match the core maven behaviour.
      *