runtime error: applying non-zero offset 4 to null pointer(stl_vector.h)
abhaynitjsr
7944

It is leetcode's question #630 - course schedule iii.
I am using priority queue and greedy approach to solve the problem. I was almost done until a weird bug showed up out of nowhere. The bug is titled above.
I am providing the code below. Pls someone help me to get rid of this bug.

typedef vector<int> VI;

bool sortbysec(const vector<int>& v1, const vector<int>& v2)
{
    return v1[1]<=v2[1];
}

class Solution {
public:
 int scheduleCourse(vector<vector<int>>& courses) {
        
        int n=courses.size();
        sort(courses.begin(),courses.end(),sortbysec);
        priority_queue<VI> pq;
        int sum=0;
        
        for(int i=0;i<n;i++)
        {
            vector<int> current=courses[i];
            if(sum+current[0]<=current[1])
            {
                pq.push(current);
                sum+=current[0];
            }else{
                if(!pq.empty())
                {
                    vector<int> v=pq.top();
                    if(v[0]>=current[0])
                    {
                        sum=sum-v[0]+current[0];
                        pq.pop();
                        pq.push(current);
                    }
                }
            }
        }
        
        return pq.size();     
    }    
};
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