#vulnerable code 
                                @Test
    public void testOnRpcReturnedRpcError() {
        final Replicator r = getReplicator();
        assertNull(r.getBlockTimer());
        final RpcRequests.AppendEntriesRequest request = createEmptyEntriesRequest();
        final RpcRequests.AppendEntriesResponse response = RpcRequests.AppendEntriesResponse.newBuilder() //
            .setSuccess(false) //
            .setLastLogIndex(12) //
            .setTerm(2) //
            .build();
        this.id.unlock();

        Replicator.onRpcReturned(this.id, Replicator.RequestType.AppendEntries, new Status(-1, "test error"), request,
            response, 0, 0, Utils.monotonicMs());
        assertEquals(r.statInfo.runningState, Replicator.RunningState.BLOCKING);
        assertNotNull(r.getBlockTimer());
    } 
                            #location 4 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                @Test
    public void testOnRpcReturnedRpcError() {
        testRpcReturnedError();
    }