[generator] Fix an NRE when cloning a method with generic arguments (#1089)

Context: #1080
Context: 9e0a469

When building an updated
`com.google.firebase.firebase-components.csproj` in
xamarin/GooglePlayServicesComponents using a *post*- 9e0a469
`generator`, the build hits an NullReferenceException when attempting
to `Method.Clone (…)` a method with generic arguments:

	error BG0000: System.NullReferenceException: Object reference not set to an instance of an object.
	   at MonoDroid.Generation.Method.Clone(GenBase declaringType) in C:\code\xamarin-android\external\Java.Interop\tools\generator\Java.Interop.Tools.Generator.ObjectModel\Method.cs:line 131
	   at MonoDroid.Generation.Method.Clone(GenBase declaringType) in C:\code\xamarin-android\external\Java.Interop\tools\generator\Java.Interop.Tools.Generator.ObjectModel\Method.cs:line 131
	   at MonoDroid.Generation.ClassGen.FixupAccessModifiers(CodeGenerationOptions opt) in C:\code\xamarin-android\external\Java.Interop\tools\generator\Java.Interop.Tools.Generator.ObjectModel\ClassGen.cs:line 67
	   at MonoDroid.Generation.ClassGen.FixupAccessModifiers(CodeGenerationOptions opt) in C:\code\xamarin-android\external\Java.Interop\tools\generator\Java.Interop.Tools.Generator.ObjectModel\ClassGen.cs:line 67
	   at Xamarin.Android.Binder.CodeGenerator.Validate(List`1 gens, CodeGenerationOptions opt, CodeGeneratorContext context) in C:\code\xamarin-android\external\Java.Interop\tools\generator\CodeGenerator.cs:line 300
	   at Xamarin.Android.Binder.CodeGenerator.Validate(List`1 gens, CodeGenerationOptions opt, CodeGeneratorContext context) in C:\code\xamarin-android\external\Java.Interop\tools\generator\CodeGenerator.cs:line 300
	   at Xamarin.Android.Binder.CodeGenerator.Run(CodeGeneratorOptions options, DirectoryAssemblyResolver resolver) in C:\code\xamarin-android\external\Java.Interop\tools\generator\CodeGenerator.cs:line 208
	   at Xamarin.Android.Binder.CodeGenerator.Run(CodeGeneratorOptions options, DirectoryAssemblyResolver resolver) in C:\code\xamarin-android\external\Java.Interop\tools\generator\CodeGenerator.cs:line 208
	   at Xamarin.Android.Binder.CodeGenerator.Run(CodeGeneratorOptions options) in C:\code\xamarin-android\external\Java.Interop\tools\generator\CodeGenerator.cs:line 50
	   at Xamarin.Android.Binder.CodeGenerator.Run(CodeGeneratorOptions options) in C:\code\xamarin-android\external\Java.Interop\tools\generator\CodeGenerator.cs:line 50
	   at Xamarin.Android.Binder.CodeGenerator.Main(String[] args) in C:\code\xamarin-android\external\Java.Interop\tools\generator\CodeGenerator.cs:line 33
	   at Xamarin.Android.Binder.CodeGenerator.Main(String[] args) in C:\code\xamarin-android\external\Java.Interop\tools\generator\CodeGenerator.cs:line 33

This is because the `MethodBase.GenericArguments` collection is `null`
unless it has items.  Thus we need to create it before populating it
with items in `Method.Clone(GenBase)`.

Author: jpobst

tests/generator-Tests/Unit-Tests/GenBaseTests.cs

@@ -91,6 +91,22 @@ public void TestEqualsMethodsWithOneParameter ()
 			Assert.False (c.RequiresNew (m.Name, m));
 		}
 
+		[Test]
+		public void TestMethodClone_GenericArguments ()
+		{
+			var c = SupportTypeBuilder.CreateClass ("java.myClass", options);
+			var m = SupportTypeBuilder.CreateMethod (c, "DoStuff", options);
+
+			m.GenericArguments = new GenericParameterDefinitionList {
+				new GenericParameterDefinition ("T", null)
+			};
+
+			var clone = m.Clone (c);
+
+			Assert.AreEqual (1, clone.GenericArguments.Count);
+			Assert.AreEqual ("T", clone.GenericArguments [0].Name);
+		}
+
 		void TestParameterlessMethods (string name)
 		{
 			var c = SupportTypeBuilder.CreateClass ("java.myClass", options);

tools/generator/Java.Interop.Tools.Generator.ObjectModel/Method.cs

@@ -126,9 +126,13 @@ public Method Clone (GenBase declaringType)
 			clone.SourceFile = SourceFile;
 			clone.JavadocInfo = JavadocInfo;
 
-			if (GenericArguments != null)
+			if (GenericArguments != null) {
+				if (clone.GenericArguments is null)
+					clone.GenericArguments = new GenericParameterDefinitionList ();
+
 				foreach (var ga in GenericArguments)
 					clone.GenericArguments.Add (ga.Clone ());
+			}
 
 			foreach (var p in Parameters)
 				clone.Parameters.Add (p.Clone ());