Callback function not working

Hello,

I'm trying to use a callback function to apply an heuristic for each solution. I am not able to do it. I write here a simplified version of the code. In my case, there may be hundreds of solutions with the same objective function so I want to use the heuristic inside the callback to select the best one. I don't know where the error is and why I get an empty list, so I can not treat the solution. 

def mycallback(model, where):

    if where == GRB.Callback.MIPSOL:
            print(model.cbGetSolution(model._vars))
            solution = model.cbGetSolution(model._vars)
            model._solutions[model._solution_count] = solution
            
            ... #Things I'd do if I had something in the variable "solution"

            model._solution_count += 1            
            
m = Model() # Gurobi model

# Parameters
# Gurobi variables 
P = m.addVars(I,I,vtype=GRB.BINARY, name="Prio_")  
# constraints

...

# objective function                                 
m.setObjective( quicksum(P[i,j] for i in I for j in I), GRB.MAXIMIZE )   

m.Params.Threads = 1

# With or without the following 2 lines, the result is the same
m.setParam('PoolSearchMode',2) 
m.setParam('PoolSolutions',10) # 

m._solution_count = 0
m._solutions = {}
m._solutions_quality = {}
m._vars = m.getVars()    

m.optimize(mycallback)

This is the output: 

Set parameter Username
Academic license - for non-commercial use only - expires 2023-09-24
Set parameter Threads to value 1
Set parameter TimeLimit to value 500
Gurobi Optimizer version 9.5.1 build v9.5.1rc2 (linux64)
Thread count: 6 physical cores, 6 logical processors, using up to 1 threads
Optimize a model with 199 rows, 81 columns and 493 nonzeros
Model fingerprint: 0x665c8cbc
Variable types: 0 continuous, 81 integer (81 binary)
Coefficient statistics:
  Matrix range     [1e+00, 1e+00]
  Objective range  [1e+00, 1e+00]
  Bounds range     [1e+00, 1e+00]
  RHS range        [1e+00, 1e+01]
[ ]  (this is the result of  print(model.cbGetSolution(model._vars)))
Found heuristic solution: objective 16.0000000
Presolve removed 179 rows and 71 columns
Presolve time: 0.00s
Presolved: 20 rows, 10 columns, 60 nonzeros
Variable types: 0 continuous, 10 integer (10 binary)

Explored 0 nodes (0 simplex iterations) in 0.00 seconds (0.00 work units)
Thread count was 1 (of 6 available processors)

Solution count 1: 16 

Optimal solution found (tolerance 1.00e-04)
Best objective 1.600000000000e+01, best bound 1.600000000000e+01, gap 0.0000%

User-callback calls 209, time in user-callback 0.00 sec