[ApiXmlAdjuster] Guard against NRE when writing generic constraints. (#495)

Context: #466
Context: #467

When attempting to bind [`kotlin-stdlib-1.3.41.jar`][0], `generator`
crashes with the following exception:

	System.NullReferenceException: Object reference not set to an instance of an object
	  at Xamarin.Android.Tools.ApiXmlAdjuster.JavaApiXmlGeneratorExtensions.Save (Xamarin.Android.Tools.ApiXmlAdjuster.JavaTypeParameters typeParameters, System.Xml.XmlWriter writer, System.String indent)
	  at Xamarin.Android.Tools.ApiXmlAdjuster.JavaApiXmlGeneratorExtensions.SaveCommon (Xamarin.Android.Tools.ApiXmlAdjuster.JavaMember m, System.Xml.XmlWriter writer, System.String elementName, System.String abs, System.String native, System.String ret, System.String sync, System.String transient, System.String type, System.String typeGeneric, System.String value, System.String volat, Xamarin.Android.Tools.ApiXmlAdjuster.JavaTypeParameters typeParameters, System.Collections.Generic.IEnumerable`1[T] parameters, System.Collections.Generic.IEnumerable`1[T] exceptions, System.Nullable`1[T] extBridge, System.String jniReturn, System.Nullable`1[T] extSynthetic)
	  at Xamarin.An/droid.Tools.ApiXmlAdjuster.JavaApiXmlGeneratorExtensions.Save (Xamarin.Android.Tools.ApiXmlAdjuster.JavaMethod method, System.Xml.XmlWriter writer)
	  at Xamarin.Android.Tools.ApiXmlAdjuster.JavaApiXmlGeneratorExtensions.SaveTypeCommon (Xamarin.Android.Tools.ApiXmlAdjuster.JavaType cls, System.Xml.XmlWriter writer, System.String elementName, System.String abs, System.String ext, System.String extgen, System.String jniExt)
	  at Xamarin.Android.Tools.ApiXmlAdjuster.JavaApiXmlGeneratorExtensions.Save (Xamarin.Android.Tools.ApiXmlAdjuster.JavaClass cls, System.Xml.XmlWriter writer)
	  at Xamarin.Android.Tools.ApiXmlAdjuster.JavaApiXmlGeneratorExtensions.Save (Xamarin.Android.Tools.ApiXmlAdjuster.JavaApi api, System.Xml.XmlWriter writer)
	  at Xamarin.Android.Tools.ApiXmlAdjuster.JavaApiXmlGeneratorExtensions.Save (Xamarin.Android.Tools.ApiXmlAdjuster.JavaApi api, System.String xmlfile)
	  at Xamarin.Android.Tools.ApiXmlAdjuster.Adjuster.Process (System.String inputXmlFile, MonoDroid.Generation.CodeGenerationOptions opt, MonoDroid.Generation.GenBase[] gens, System.String outputXmlFile, System.Int32 reportVerbosity)
	  at Xamarin.Android.Binder.CodeGenerator.Run (Xamarin.Android.Binder.CodeGeneratorOptions options, Java.Interop.Tools.Cecil.DirectoryAssemblyResolver resolver)
	  at Xamarin.Android.Binder.CodeGenerator.Run (Xamarin.Android.Binder.CodeGeneratorOptions options)
	  at Xamarin.Android.Binder.CodeGenerator.Main (System.String[] args)

We are still investigating *why* this happens and how it should be
appropriately fixed.

In the meantime, to "unblock" developers attempting to bind binding
Kotlin libraries in Visual Studio 16.4 Preview 2, simply prevent the
`NullReferenceException` from being generated generated.  This at
least lets people get past the crash so they can apply fixups in
`Metadata.xml` to continue.  (This `NullReferenceException` happens
before the `metadata` stage; there's nothing a user can currently do.)

I will be making our Kotlin bindings more "correct" in future sprints.

[0]: https://search.maven.org/remotecontent?filepath=org/jetbrains/kotlin/kotlin-stdlib/1.3.41/kotlin-stdlib-1.3.41.jar

Author: jpobst

src/Xamarin.Android.Tools.ApiXmlAdjuster/JavaApiXmlGeneratorExtensions.cs

@@ -124,7 +124,7 @@ static void Save (this JavaTypeParameters typeParameters, XmlWriter writer, stri
 					// jar2xml does not emit that either.
 					var gcs = tp.GenericConstraints.GenericConstraints;
 					var gctr = gcs.Count == 1 ? gcs [0].ResolvedType : null;
-					if (gctr == null || gctr.ReferencedType.FullName != "java.lang.Object")
+					if (gctr?.ReferencedType?.FullName != "java.lang.Object")
 					{
 						writer.WriteStartElement ("genericConstraints");
 						foreach (var g in tp.GenericConstraints.GenericConstraints) {