Why Am I Getting NullPointerException When I Run the HTML Playn Samples?

Question

I'm trying to run the Playn html web application samples in Eclipse. I've followed the steps here:

http://code.google.com/p/playn/wiki/GettingStarted#Running_via_Eclipse

I'm running Eclipse 3.7 on Ubuntu 11.10. I installed the plugins and I was able to run the Java version successfully.

However, when I run GWT Compile by right-clicking the html listing in the Package Explorer, I get the following error:

An internal error occurred during: "playn-showcase-html - GWT Compile". java.lang.NullPointerException

This occurs with the other samples too. When I get to the GWT Compile dialogue box, a message at top says "The project has errors."

When I start Eclipse, I get the following message in the console:

GWT SDK not installed.

However, the SDK is installed. I double-checked by redoing the steps here:

http://code.google.com/intl/da-DK/eclipse/docs/install-eclipse-3.7.html

And I get the following message:

"Google Web Toolkit SDK 2.4.0" will be ignored because it is already installed.