Exception thrown at 0x00007FF6E384A096 in NewTest.exe: 0xC0000005: Access violation reading location 0x0000000000000008

I don't know why I am getting this error. I tried to use breakpoint and found out it gives me the error when it wants to go through the 4th for loop (for tt=0...).

//Constraints 12
for (cc = 0; cc < NumberOfCourses; cc++)
    for (ww = 0; ww < AvailableWeeks; ww++) {
        IloExpr Constraint12;
        for (rr = 0; rr < AvailableRooms; rr++)
            for (tt = 0; tt < AvailableTimeSlots; tt++) { //error line
                Constraint12 += Z[cc][rr][tt][ww];
            }
        mod.add(Constraint12 = AN[cc][ww]);
        Constraint12.end();
    }