#vulnerable code 
                                private void recalc(Block bestBlock) {
        this.currentRound = null;
        this.previousRound = null;
        this.calc(bestBlock);
    } 
                            #location 4 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                private void recalc(Block bestBlock) {
        this.currentRound = null;
        this.calc(bestBlock);
    }