As noted here Issue 4220 – google-web-toolkit – GWT crash (Safari 4.0.4 … and Issue 4091 – google-web-toolkit – Invalid memory access of …, Safar 4.0.4 breaks GWT’s hosted mode.

You can modify a check within GWT as found here (link broken) or you can just downgrade Safari to 4.0.3.

The files attached come from my MacBook Pro running 10.6.2. I retrieved the files by looking at what was installed in the Safari 4.0.4 update by using Pacifist and then copied the older versions from a backup.

These files are just for Snow Leopard, if you are using Leopard you should just be able to downgrade using the installer (Safari 4.0.3 Leopard Installer) or the extracted files Safari Downgrade (Leopard)

The files attached are organised as if the folder was your root volume.

Safari Downgrade (Snow Leopard)

UPDATE: Much better solution here. You won’t have to downgrade. You just have to set an environment variable (see the above post)