more helpful error message

git-svn-id: https://svn.apache.org/repos/asf/maven/components/trunk@165173 13f79535-47bb-0310-9956-ffa450edef68

maven-artifact/src/main/java/org/apache/maven/artifact/DefaultArtifact.java

@@ -240,6 +240,11 @@ public class DefaultArtifact
         if ( baseVersion == null )
         {
             baseVersion = version;
+
+            if ( version == null )
+            {
+                throw new NullPointerException( "version was null for " + groupId + ":" + artifactId );
+            }
         }
         return baseVersion;
     }
@@ -289,4 +294,4 @@ public class DefaultArtifact
         }
         return result;
     }
-}
+}