[ObjCRuntime] Enable nullability and clean up BackingField.

src/ObjCRuntime/BackingField.cs

@@ -37,15 +37,23 @@
 
 #if IOS
 
-// Disable until we get around to enable + fix any issues.
-#nullable disable
+#nullable enable
 
 namespace ObjCRuntime {
 
+	/// <summary>Helpers to convert between native handles and cached managed wrappers.</summary>
 	static class BackingField {
-		public static T Get<T> (ref T value, IntPtr handle, Converter<IntPtr, T> c)
+		/// <summary>Gets a cached managed wrapper or creates a new one from a native handle.</summary>
+		/// <typeparam name="T">The managed wrapper type.</typeparam>
+		/// <param name="value">The cached managed value.</param>
+		/// <param name="handle">The native handle.</param>
+		/// <param name="c">A converter that creates a managed wrapper from a native handle.</param>
+		/// <returns>The cached or newly created managed value, or <see langword="null"/> if <paramref name="handle"/> is <see cref="IntPtr.Zero"/>.</returns>
+		public static T? Get<T> (ref T? value, IntPtr handle, Converter<IntPtr, T?> c)
 			where T : class, INativeObject, IDisposable
 		{
+			c = ThrowHelper.ThrowIfNull (c);
+
 			if (handle == IntPtr.Zero) {
 				if (value is not null)
 					value.Dispose ();
@@ -59,7 +67,12 @@ public static T Get<T> (ref T value, IntPtr handle, Converter<IntPtr, T> c)
 			return value = c (handle);
 		}
 
-		public static IntPtr Save<T> (ref T value, T newValue)
+		/// <summary>Saves a managed wrapper in a backing field and returns the corresponding native handle.</summary>
+		/// <typeparam name="T">The managed wrapper type.</typeparam>
+		/// <param name="value">The current cached managed value.</param>
+		/// <param name="newValue">The new managed value to cache.</param>
+		/// <returns>The native handle for <paramref name="newValue"/>, or <see cref="IntPtr.Zero"/> if <paramref name="newValue"/> is <see langword="null"/>.</returns>
+		public static IntPtr Save<T> (ref T? value, T? newValue)
 			where T : class, INativeObject, IDisposable
 		{
 #pragma warning disable RBI0014