Make ThreadSafeObjectPool actually thread safe (#8106)

* Make ThreadSafeObjectPool actually thread safe

src/Avalonia.Base/Threading/ThreadSafeObjectPool.cs

@@ -5,12 +5,11 @@ namespace Avalonia.Threading
     public class ThreadSafeObjectPool<T> where T : class, new()
     {
         private Stack<T> _stack = new Stack<T>();
-        private object _lock = new object();
         public static ThreadSafeObjectPool<T> Default { get; } = new ThreadSafeObjectPool<T>();
 
         public T Get()
         {
-            lock (_lock)
+            lock (_stack)
             {
                 if(_stack.Count == 0)
                     return new T();