[tools] Simplify the creation of the list of frameworks a bit.

scripts/generate-frameworks-constants/generate-frameworks-constants.cs

@@ -32,7 +32,10 @@ static ApplePlatform GetPlatform (string platform)
 
 		static int Fix (ApplePlatform platform, string output)
 		{
-			var frameworks = Frameworks.GetFrameworks (platform, false)!.Values.Where (v => !v.IsFrameworkUnavailable ());
+			if (!Frameworks.TryGetFrameworks (platform, out var fwks))
+				return 1;
+
+			var frameworks = fwks.Values.Where (v => !v.IsFrameworkUnavailable ());
 			var sb = new StringBuilder ();
 
 			sb.AppendLine ("namespace ObjCRuntime {");

src/rgen/Microsoft.Macios.Bindings.Analyzer/Validators/SmartEnumValidator.cs

@@ -134,8 +134,7 @@ internal static bool ValidateLibraryPathValue (Binding binding, RootContext cont
 			return false;
 		}
 
-		var appleFrameworks = Frameworks.GetFrameworks (platformName.ToApplePlatform (), false);
-		if (appleFrameworks is null) {
+		if (!Frameworks.TryGetFrameworks (platformName.ToApplePlatform (), out var appleFrameworks)) {
 			// we could not get the frameworks, we have a bug
 			// we could not identify the platform, we have a bug 
 			diagnostics = [Diagnostic.Create (

tools/common/Driver.cs

@@ -688,8 +688,7 @@ public static string CorlibName {
 
 		public static Frameworks GetFrameworks (Application app)
 		{
-			var rv = Frameworks.GetFrameworks (app.Platform, app.IsSimulatorBuild);
-			if (rv is null)
+			if (!Frameworks.TryGetFrameworks (app.Platform, out var rv))
 				throw ErrorHelper.CreateError (71, Errors.MX0071, app.Platform, app.ProductName);
 			return rv;
 		}