CpSolver.StopSearch() Dead Lock

**What version of OR-Tools and what language are you using?**
Version: 9.14.6206
Language: C#

**Which solver are you using (e.g. CP-SAT, Routing Solver, GLOP, BOP, Gurobi)**
CP-SAT

**What operating system (Linux, Windows, ...) and version?**
Windows

**What did you do?**
Steps to reproduce the behavior:
1. Create solver: var solver = new CpSolver();
2. Slove Model: var status = solver.Solve(model, cb);
3. Call solver.StopSearch() by another thread
4. dead lock: StopSearch() do not return, SolveWrapper.Dispose() wait StopSearch() Thread free lock

**What did you expect to see**

**What did you see instead?**

Make sure you include information that can help us debug (full error message, model Proto).

**Anything else we should know about your project / environment**