The use case is exactly the same as described in RT-37549 when the packager is called to produce a DMG from an APP (the APP being constructed with the packager too).
I gave a try to 8 U20 b19 to benefit the fix of RT-37549: things are way better but still it fails.

The failure occurs when evaluating the scpt script (I use default one):
  /var/folders/2h/f4qpfbnn0n18s12l93tphzh000grx0/T/fxbundler1622062350003254700/macosx/JavaFX Scene Builder 2.1-dmg-setup.scpt:1271:1352: execution error: Finder got an error: Can’t set item "JavaFX Scene Builder 2.1" of Finder window id 8434 to {120, 135}. (-10006)

The scpt line in question is:
  set position of item "JavaFX Scene Builder 2.1" of container window to {120, 135}

The reason of the failure is that the APP is mounted with name "JavaFX Scene Builder 2.1.app".
I copied the default scpt to use it as a custom one and modified the failing line to:
  set position of item "JavaFX Scene Builder 2.1.app" of container window to {120, 135}

The failure goes away but problem is that the user reads an application name ending with ".app" in the DMG installation window, something which wasn't the case with FX 8 GA nor U20 b02.
I suspect the default scpt script is correct but what's perhaps wrong is the app: why is it now "seen" with a name ending by ".app" during the APP --> DMG step ?