#vulnerable code 
                                private void visitInvoke(InvokeInstruction obj) {
        assert obj != null;
        try {
            TaintMethodConfig methodConfig = getMethodConfig(obj);
            ObjectType realInstanceClass = (methodConfig == null) ?
                    null : methodConfig.getOutputTaint().getRealInstanceClass();
            Taint taint = getMethodTaint(methodConfig);
            assert taint != null;
            if (FindSecBugsGlobalConfig.getInstance().isDebugTaintState()) {
                taint.setDebugInfo(obj.getMethodName(cpg) + "()"); //TODO: Deprecated debug info
            }
            taint.addSource(new UnknownSource(UnknownSourceType.RETURN,taint.getState()).setSignatureMethod(obj.getClassName(cpg).replace(".","/")+"."+obj.getMethodName(cpg)+obj.getSignature(cpg)));
            if (taint.isUnknown()) {
                taint.addLocation(getTaintLocation(), false);
            }
            taintMutableArguments(methodConfig, obj);
            transferTaintToMutables(methodConfig, taint); // adds variable index to taint too
            Taint taintCopy = new Taint(taint);
            // return type is not always the instance type
            taintCopy.setRealInstanceClass(realInstanceClass);

            TaintFrame tf = getFrame();

            int stackDepth = tf.getStackDepth();
            int nbParam = getNumWordsConsumed(obj);
            List<Taint> parameters = new ArrayList<>(nbParam);
            for(int i=0;i<Math.min(stackDepth,nbParam);i++) {
                parameters.add(new Taint(tf.getStackValue(i)));
            }

            modelInstruction(obj, getNumWordsConsumed(obj), getNumWordsProduced(obj), taintCopy);

            for(TaintFrameAdditionalVisitor visitor : visitors) {
                try {
                    visitor.visitInvoke(obj, methodGen, getFrame() , parameters, cpg);
                }
                catch (Throwable e) {
                    LOG.log(Level.SEVERE,"Error while executing "+visitor.getClass().getName(),e);
                }
            }

        } catch (Exception e) {
            String className = ClassName.toSlashedClassName(obj.getReferenceType(cpg).toString());
            String methodName = obj.getMethodName(cpg);
            String signature = obj.getSignature(cpg);

            throw new RuntimeException("Unable to call " + className + '.' + methodName + signature, e);
        }
    } 
                            #location 6 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                private void visitInvoke(InvokeInstruction obj) {
        assert obj != null;
        try {
            TaintMethodConfig methodConfig = getMethodConfig(obj);
            Taint taint = getMethodTaint(methodConfig);
            assert taint != null;
            if (FindSecBugsGlobalConfig.getInstance().isDebugTaintState()) {
                taint.setDebugInfo(obj.getMethodName(cpg) + "()"); //TODO: Deprecated debug info
            }
            taint.addSource(new UnknownSource(UnknownSourceType.RETURN,taint.getState()).setSignatureMethod(obj.getClassName(cpg).replace(".","/")+"."+obj.getMethodName(cpg)+obj.getSignature(cpg)));
            if (taint.isUnknown()) {
                taint.addLocation(getTaintLocation(), false);
            }
            taintMutableArguments(methodConfig, obj);
            transferTaintToMutables(methodConfig, taint); // adds variable index to taint too
            Taint taintCopy = new Taint(taint);
            // return type is not always the instance type
            taintCopy.setRealInstanceClass(methodConfig != null && methodConfig.getOutputTaint() != null ? methodConfig.getOutputTaint().getRealInstanceClass() : null);

            TaintFrame tf = getFrame();

            int stackDepth = tf.getStackDepth();
            int nbParam = getNumWordsConsumed(obj);
            List<Taint> parameters = new ArrayList<>(nbParam);
            for(int i=0;i<Math.min(stackDepth,nbParam);i++) {
                parameters.add(new Taint(tf.getStackValue(i)));
            }

            modelInstruction(obj, getNumWordsConsumed(obj), getNumWordsProduced(obj), taintCopy);

            for(TaintFrameAdditionalVisitor visitor : visitors) {
                try {
                    visitor.visitInvoke(obj, methodGen, getFrame() , parameters, cpg);
                }
                catch (Throwable e) {
                    LOG.log(Level.SEVERE,"Error while executing "+visitor.getClass().getName(),e);
                }
            }

        } catch (Exception e) {
            String className = ClassName.toSlashedClassName(obj.getReferenceType(cpg).toString());
            String methodName = obj.getMethodName(cpg);
            String signature = obj.getSignature(cpg);

            throw new RuntimeException("Unable to call " + className + '.' + methodName + signature, e);
        }
    }