getValue() CPLEX C++ : Using empty IloNumVar handle

I try to get the values of variables (IloNumVar) in an LP. The LP is solved successfully, but I cannot access the variables in the LP. The solver prints "Concert exception caught:Using empty IloNumVar handle." for the getValue() function.

This is my code for initializing the varialbes and accessing the variables in LP

        //define variables
        IloArray<IloArray<IloNumVarArray> > x(env, uNum);
        for(k=0; k<uNum; k++)
        {
            x[k]=IloArray<IloNumVarArray> (env, nNum);
            for(j=0; j<nNum; j++)
            {
                x[k][j]=IloNumVarArray(env, nNum, 0, 1, ILOFLOAT);
            }
        }

         ...

         //get values of variables
         for(k=0; k<uNum; k++)
         {
              for(j=0; j<nNum; j++)
              {
                   for(l=0; l<nNum; l++)
                   {
                        cout<<cplex.getValue(x[k][j][l])<<endl;
                            
                   }
              }
         }

The output is as follows.

Version identifier: 20.1.0.0 | 2020-11-10 | 9bedb6d68
Parallel mode: deterministic, using up to 32 threads for concurrent optimization:
 * Starting dual Simplex on 1 thread...
 * Starting Barrier on 29 threads...
 * Starting primal Simplex on 1 thread...
 * Starting Sifting on 1 thread...
Tried aggregator 1 time.
LP Presolve eliminated 2 rows and 0 columns.
Reduced LP has 140 rows, 44100 columns, and 128100 nonzeros.
Presolve time = 0.10 sec. (32.34 ticks)
Initializing dual steep norms . . .

Iteration log . . .
Iteration:     1   Dual objective     =           160.209490

Dual simplex solved model.

Solution status: Optimal
The objective value is 160.209
Concert exception caught:Using empty IloNumVar handle.

I found that someone asked similar question for getValue() function (getValue() cplex C++). I checked the answer and my code, and still cannot solve the issue. Please help if you have any suggestions.

Looks like one of the variables is not constrained, thus, not "extracted" by the cplex solver, and it cannot give the value of a variable it doesn't know. You can simply add all the variables to the model.