Product Version = NetBeans IDE Dev (Build 201312300942)
Operating System = Windows 7 version 6.1 running on amd64
Java; VM; Vendor = 1.8.0-ea
Runtime = Java HotSpot(TM) 64-Bit Server VM 25.0-b63

Install NB with JDK 8 as the default platform. Also install JDK 7u45.
Create a new JavaFX application.
Keep default "Hello World" app.
Compile and run -> success.
Change the Java Platform for the project from the default to JDK 7u45 (adding it as a Java Platform if needed).
Compile and run the application. 
Result:

Exception in thread "main" java.lang.NoClassDefFoundError: javafx/application/Application
        at java.lang.ClassLoader.defineClass1(Native Method)
        at java.lang.ClassLoader.defineClass(Unknown Source)
        at java.security.SecureClassLoader.defineClass(Unknown Source)
        at java.net.URLClassLoader.defineClass(Unknown Source)
        at java.net.URLClassLoader.access$100(Unknown Source)
        at java.net.URLClassLoader$1.run(Unknown Source)
        at java.net.URLClassLoader$1.run(Unknown Source)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(Unknown Source)
        at java.lang.ClassLoader.loadClass(Unknown Source)
        at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
        at java.lang.ClassLoader.loadClass(Unknown Source)
        at sun.launcher.LauncherHelper.checkAndLoadMain(Unknown Source)
Caused by: java.lang.ClassNotFoundException: javafx.application.Application
        at java.net.URLClassLoader$1.run(Unknown Source)
        at java.net.URLClassLoader$1.run(Unknown Source)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(Unknown Source)
        at java.lang.ClassLoader.loadClass(Unknown Source)
        at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
        at java.lang.ClassLoader.loadClass(Unknown Source)
        ... 13 more

The application is incorrectly packaged and does not augment the classpath with the JavaFX runtime.