#vulnerable code 
                                public Block getHighestBlock() {
        BlockHeaderChain chain = bifurcateProcessor.getApprovingChain();
        if (null == chain) {
            return null;
        }
        HeaderDigest headerDigest = chain.getLastHd();
        return this.getBlock(headerDigest.getHash());
    } 
                            #location 7 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                public Block getHighestBlock() {
        BlockHeaderChain chain = bifurcateProcessor.getApprovingChain();
        if (null == chain) {
            return null;
        }
        HeaderDigest headerDigest = chain.getLastHd();
        if(null==headerDigest){
            return null;
        }
        return this.getBlock(headerDigest.getHash());
    }