Merge pull request #1352 from dotnet/dev/bartde/rx_nullable_step13

Enable #nullable for a few more scheduler types.

Author: bartdesmet

Rx.NET/Source/src/System.Reactive/Platforms/Desktop/Concurrency/ControlScheduler.cs

@@ -1,8 +1,6 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT License.
-// See the LICENSE file in the project root for more information. 
-
-#nullable disable
+// See the LICENSE file in the project root for more information.
 
 using System.Reactive.Disposables;
 using System.Threading;
@@ -33,10 +31,7 @@ public ControlScheduler(Control control)
         /// <summary>
         /// Gets the control associated with the ControlScheduler.
         /// </summary>
-        public Control Control
-        {
-            get { return _control; }
-        }
+        public Control Control => _control;
 
         /// <summary>
         /// Schedules an action to be executed on the message loop associated with the control.
@@ -114,7 +109,7 @@ public override IDisposable Schedule<TState>(TState state, TimeSpan dueTime, Fun
                         finally
                         {
                             t.Stop();
-                            action = null;
+                            action = static (s, t) => Disposable.Empty;
                         }
                     }
                 };

Rx.NET/Source/src/System.Reactive/Platforms/Desktop/Concurrency/DispatcherScheduler.cs

@@ -2,8 +2,6 @@
 // The .NET Foundation licenses this file to you under the MIT License.
 // See the LICENSE file in the project root for more information. 
 
-#nullable disable
-
 #if HAS_WPF
 using System.Reactive.Disposables;
 using System.Threading;
@@ -149,7 +147,7 @@ private IDisposable ScheduleSlow<TState>(TState state, TimeSpan dueTime, Func<IS
                     finally
                     {
                         t.Stop();
-                        action = null;
+                        action = static (s, t) => Disposable.Empty;
                     }
                 }
             };

Rx.NET/Source/src/System.Reactive/Platforms/UWP/Concurrency/CoreDispatcherScheduler.cs

@@ -2,8 +2,6 @@
 // The .NET Foundation licenses this file to you under the MIT License.
 // See the LICENSE file in the project root for more information. 
 
-#nullable disable
-
 #if WINDOWS
 using System.Reactive.Disposables;
 using System.Runtime.ExceptionServices;
@@ -200,7 +198,7 @@ private IDisposable ScheduleSlow<TState>(TState state, TimeSpan dueTime, Func<IS
                     finally
                     {
                         t.Stop();
-                        action = null;
+                        action = static (s, t) => Disposable.Empty;
                     }
                 }
             };