[One .NET] fix code path for unhandled exceptions (#4878)

Context: dotnet/runtime#38465

When a thread throws an exception which isn't caught, we (eventually)
hit the "unhandled exception" codepath of
`JNIEnv.PropagateUncaughtException()` (2aff4e7 & others), which
relies on the following methods which are not present in .NET 5:

  * `System.Diagnostics.Debugger.Mono_UnhandledException()`
  * `AppDomain.DoUnhandledException()`

Unfortunately, we didn't check that the `MethodInfo`s for these
methods were `null` before invoking them, which means under .NET 5 we
would throw an exception while attempting to deal with an already
"in-flight" exception, i.e. we'd throw a *nested* exception!

	Unable to initialize UncaughtExceptionHandler. Nested exception caught: System.ArgumentNullException: Value cannot be null. (Parameter 'method')
	    at System.Delegate.CreateDelegate(Type type, Object firstArgument, MethodInfo method, Boolean throwOnBindFailure, Boolean allowClosed)
	    at System.Delegate.CreateDelegate(Type type, MethodInfo method, Boolean throwOnBindFailure)
	    at System.Delegate.CreateDelegate(Type type, MethodInfo method)
	    at Android.Runtime.JNIEnv.Initialize()
	    at Android.Runtime.JNIEnv.PropagateUncaughtException(IntPtr env, IntPtr javaThreadPtr, IntPtr javaExceptionPtr)

or:

	Exception thrown while raising AppDomain.UnhandledException event: System.NullReferenceException: Object reference not set to an instance of an object
	    at Android.Runtime.JNIEnv.PropagateUncaughtException(IntPtr env, IntPtr javaThreadPtr, IntPtr javaExceptionPtr)

Add appropriate `null` checks in `JNIEnv.PropagateUncaughtException()`
to avoid generation of nested exceptions.

TODO: Work with the dotnet/mono teams so that we can reintroduce the
functionality of `AppDomain.DoUnhandledException()`/etc..

src/Mono.Android/Android.Runtime/JNIEnv.cs

@@ -244,8 +244,9 @@ static void Initialize ()
 		{
 			if (mono_unhandled_exception == null) {
 				var mono_UnhandledException = typeof (System.Diagnostics.Debugger)
-					.GetMethod ("Mono_UnhandledException", BindingFlags.NonPublic | BindingFlags.Static)!;
-				mono_unhandled_exception = (Action<Exception>) Delegate.CreateDelegate (typeof(Action<Exception>), mono_UnhandledException);
+					.GetMethod ("Mono_UnhandledException", BindingFlags.NonPublic | BindingFlags.Static);
+				if (mono_UnhandledException != null)
+					mono_unhandled_exception = (Action<Exception>) Delegate.CreateDelegate (typeof(Action<Exception>), mono_UnhandledException);
 			}
 
 			if (AppDomain_DoUnhandledException == null) {
@@ -277,7 +278,7 @@ internal static void PropagateUncaughtException (IntPtr env, IntPtr javaThreadPt
 
 			// Disabled until Linker error surfaced in https://github.com/xamarin/xamarin-android/pull/4302#issuecomment-596400025 is resolved
 			//System.Diagnostics.Debugger.Mono_UnhandledException (javaException);
-			mono_unhandled_exception (javaException);
+			mono_unhandled_exception?.Invoke (javaException);
 
 			try {
 				var jltp = javaException as JavaProxyThrowable;
@@ -289,7 +290,7 @@ internal static void PropagateUncaughtException (IntPtr env, IntPtr javaThreadPt
 
 				// Disabled until Linker error surfaced in https://github.com/xamarin/xamarin-android/pull/4302#issuecomment-596400025 is resolved
 				//AppDomain.CurrentDomain.DoUnhandledException (args);
-				AppDomain_DoUnhandledException (AppDomain.CurrentDomain, args);
+				AppDomain_DoUnhandledException?.Invoke (AppDomain.CurrentDomain, args);
 			} catch (Exception e) {
 				Logger.Log (LogLevel.Error, "monodroid", "Exception thrown while raising AppDomain.UnhandledException event: " + e.ToString ());
 			}