The plugins might be conflicting.And why not directly use an Eclipse for RCP and RAP Developers version.The WindowBuilder plugin is included and you don't need to download it.
Eclipse for RCP and RAP Developers :
A complete set of tools for developers who want to create Eclipse plug-ins and Rich Clients (RCP), or develop with the Remote Application Platform (RAP), plus Mylyn, XML editors, Code Recommenders, Git, CVS, and Maven integration.