fix UITestCleanup error

src/common/UITestAutomation/SessionHelper.cs

@@ -71,6 +71,7 @@ namespace Microsoft.PowerToys.UITest
             try
             {
                 appDriver?.Kill();
+                appDriver?.WaitForExit();
             }
             catch (Exception ex)
             {

src/common/UITestAutomation/UITestBase.cs

@@ -31,11 +31,6 @@ namespace Microsoft.PowerToys.UITest
             this.Session = new Session(this.sessionHelper.GetRoot(), this.sessionHelper.GetDriver());
         }
 
-        ~UITestBase()
-        {
-            this.sessionHelper.Cleanup();
-        }
-
         /// <summary>
         /// Initializes the test.
         /// </summary>
@@ -53,6 +48,15 @@ namespace Microsoft.PowerToys.UITest
             }
         }
 
+        /// <summary>
+        /// Initializes the test.
+        /// </summary>
+        [TestCleanup]
+        public void TestCleanup()
+        {
+            this.sessionHelper.Cleanup();
+        }
+
         /// <summary>
         /// Finds an element by selector.
         /// Shortcut for this.Session.Find<T>(by, timeoutMS)