In NetBeans 7.4 the FX-enabled platforms have been removed, while standard Java Platforms can now be used in FX Applications instead (because JavaFX RT is distributed as part of JDK since JDK 7u6). This change requires an auto-update mechanism to be employed when opening especially older projects in NetBeans 7.4.
Now when an old project is opened with platform set to Default_JavaFX_Platform, its platform is automatically changed to the default Java Platform. The respective classpath extension properties (in particular javac.classpath) are updated as well to keep the required FX RT artifacts on classpath if needed (although JavaFX RT is distributed together with JDK since 7u6, it is not always on classpath by default, what calls for the above workaround on NetBeans side). The respective project properties are updated also whenever user changes project platform from the UI.
The FX RT platform auto-update mechanism is now available also to SE projects for users who use FX classes in Swing and used to add jfxrt.jar dependence manually. Instead, in SE project it is now better to check "Keep JavaFX RT on classpat" checkbox in Deployment category in SE Project Properties. This keeps the correct RT version on classpath even when platform is later changed.