It should be:
_jsObjPtr= objectFromJavaScript.convert_cast<FB::JSObjectPtr>();
Any object coming from javascript is a JSObjectPtr; if it's doing a convert_cast to JSAPIPtr it will try to find inside the JSObjectPtr a C++ JSAPI object that you have created (for example, a DicomAPI object) which won't actually work on most browsers because they hide it too deep to get back out.