Looks like this has already been fixed in 3.0.0.M2.
lines 48 and 58 of DwtControl.
js are doing the same thing twice:
----------------
if (parent != null && !(parent instanceof DwtComposite))
throw new DwtException("Parent must be a subclass of Composite", DwtException.INVALIDPARENT, "DwtWidget");
....
if (!(parent instanceof DwtComposite))
throw new DwtException("DwtControl parent must be a DwtComposite", DwtException.INVALIDPARENT, "DwtControl");