#vulnerable code 
                                private void visitInvoke(InvokeInstruction obj) {
        assert obj != null;
        TaintMethodSummary methodSummary = getMethodSummary(obj);
        Taint taint = getMethodTaint(methodSummary);
        assert taint != null;
        if (taint.isUnknown()) {
            taint.addTaintLocation(getTaintLocation(), false);
        }
        taintMutableArguments(methodSummary, obj);
        transferTaintToMutables(methodSummary, taint); // adds variable index to taint too
        modelInstruction(obj, getNumWordsConsumed(obj), getNumWordsProduced(obj), new Taint(taint));
    } 
                            #location 4 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                private void visitInvoke(InvokeInstruction obj) {
        assert obj != null;
        TaintMethodSummary methodSummary = getMethodSummary(obj);
        Taint taint = getMethodTaint(methodSummary);
        assert taint != null;
        if (taint.isUnknown()) {
            taint.addLocation(getTaintLocation(), false);
        }
        taintMutableArguments(methodSummary, obj);
        transferTaintToMutables(methodSummary, taint); // adds variable index to taint too
        modelInstruction(obj, getNumWordsConsumed(obj), getNumWordsProduced(obj), new Taint(taint));
    }