Build Issue: Execution failed for task ':rosjava_messages:generateSources'.

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While trying to perform the java installation listed here: http://docs.rosjava.googlecode.com/hg/rosjava_core/html/building.html

I ran into the following issue:

:rosjava_messages:generateSources Exception in thread "main" java.lang.NullPointerException at org.ros.internal.message.GenerateInterfaces.main(GenerateInterfaces.java:162)

FAILURE: Build failed with an exception.

• What went wrong: Execution failed for task ':rosjava_messages:generateSources'. > Command '/Library/Java/JavaVirtualMachines/jdk1.7.0_17.jdk/Contents/Home/bin/java' finished with (non-zero) exit value 1.

Here is line 162 (GenerateInterfaces.java:162):

160     String rosPackagePath = System.getenv(EnvironmentVariables.ROS_PACKAGE_PATH);
161     Collection<File> packagePath = Lists.newArrayList();
162     for (String path : rosPackagePath.split(File.pathSeparator)) {
163       File packageDirectory = new File(path);
164       if (packageDirectory.exists()) {
165         packagePath.add(packageDirectory);
166       }
167     }