Why does the similar logic C# code get runtime error while another get accepted

Question

433. Minimum Genetic Mutation problem link: https://leetcode.com/problems/minimum-genetic-mutation/

I used BFS to solve this problem in C#, having no idea how does the following C# code got runtime error on LeetCode. (I have checked that this code works in C/C++) The problem seems to have something wrong inside BFS. I have tried another way to code in C# and it got accepted, but in my thought these two code are in the same logic, how does the first code lead to runtime error?

Runtime Error Message: Unhandled exception. System.NullReferenceException: Object reference not set to an instance of an object.
At Solution.MinMutation(String start, String end, String[] bank)
At __Driver__.Main(String[] args)
Last executed input: "AACCGGTT"
"AACCGGTA"
["AACCGGTA"]

RE Code

public class Solution {
    public int MinMutation(string start, string end, string[] bank) {
        Queue<int> q = new Queue<int>();
        int eid = -1;
        int[] dis = new int[bank.Length];
        for(int i = 0; i < bank.Length; i ++) {
            if(end == bank[i]) eid = i;
            dis[i] = -1;
        }
        if(eid == -1) return -1;
        q.Enqueue(-1);
        while(q.Count > 0) {
            int tp = q.Dequeue();
            for(int i = 0; i < bank.Length; i ++) {
                if(dis[i] != -1) continue;
                int cnt = 0;
                for(int j = 0; j < 8; j ++) {
                    if(tp == -1) {
                        if(bank[i][j] != start[j]) cnt ++;
                    }
                    else {
                        if(tp >= 0 && tp < bank.Length) {
                            if(bank[i][j] != bank[tp][j]) cnt ++;
                        }
                    }
                    
                }
                if(cnt == 1) {
                    dis[i] = (tp == -1? 0 : dis[tp]) + 1;
                    q.Enqueue(i);
                }
            }
        }
        return dis[eid];
    }
}

AC Code (BFS Part)

while(q.Count > 0) {
    int tp = q.Dequeue();
    for(int i = 0; i < bank.Length; i ++) {
        if(dis[i] != -1) continue;
        int cnt = 0;
        if(tp == -1) {
            for(int j = 0; j < 8; j ++) {
                if(bank[i][j] != start[j]) cnt ++;
            }
        }
        else {
            for(int j = 0; j < 8; j ++) {
                if(tp >= 0 && tp < bank.Length) {
                    if(bank[i][j] != bank[tp][j]) cnt ++;
                }
            }
        }
        if(cnt == 1) {
            dis[i] = (tp == -1? 0 : dis[tp]) + 1;
            q.Enqueue(i);
        }
    }
}