[foundation] Enable nullable for MonoTouchException

and avoid potential `NullReferenceException` in some code paths.

Also remove multiple string concatenations by using `StringBuilder`

src/Foundation/MonoTouchException.cs

@@ -1,10 +1,13 @@
 #if !MONOMAC
 
 using System;
+using System.Text;
+
+#nullable enable
 
 namespace Foundation {
 	public class MonoTouchException : Exception {
-		private NSException native_exc;
+		NSException? native_exc;
 
 		public MonoTouchException () : base () {
 			native_exc = new NSException ("default", String.Empty, null);
@@ -14,49 +17,51 @@ public MonoTouchException (NSException exc) : base () {
 			native_exc = exc;
 		}
 
-		public NSException NSException {
+		public NSException? NSException {
 			get {
 				return native_exc;
 			}
 		}
 
-		public string Reason {
+		public string? Reason {
 			get {
-				return native_exc.Reason;
+				return native_exc?.Reason;
 			}
 		}
 
-		public string Name {
+		public string? Name {
 			get {
-				return native_exc.Name;
+				return native_exc?.Name;
 			}
 		}
 
 		public override string Message {
 			get {
-				return string.Format ("Objective-C exception thrown.  Name: {0} Reason: {1}\nNative stack trace:\n{2}", Name, Reason, NativeStackTrace);
+				var sb = new StringBuilder ("Objective-C exception thrown.  Name: ").Append (Name);
+				sb.Append (" Reason: ").AppendLine (Reason);
+				sb.AppendLine ("Native stack trace:");
+				AppendNativeStackTrace (sb);
+				return sb.ToString ();
 			}
 		}
 
-		string NativeStackTrace {
-			get {
-				var symbols = native_exc.CallStackSymbols;
-				var msg = string.Empty;
-				for (int i = 0; i < symbols.Length; i++) {
-					msg += "\t" + symbols [i] + "\n";
-				}
-				return msg;
+		void AppendNativeStackTrace (StringBuilder sb)
+		{
+			if (native_exc is not null) {
+				foreach (var symbol in native_exc.CallStackSymbols)
+					sb.Append ('\t').AppendLine (symbol);
 			}
 		}
 
 		public override string ToString ()
 		{
 			var msg = base.ToString ();
+			if (native_exc is null)
+				return msg;
 
-			if (native_exc != null)
-				msg += NativeStackTrace;
-
-			return msg;
+			var sb = new StringBuilder (msg);
+			AppendNativeStackTrace (sb);
+			return sb.ToString ();
 		}
 	}
 }