[Xamarin.Android.Build.Tasks] NRE getting android.jar path (#976)

Fixes: https://bugzilla.xamarin.com/show_bug.cgi?id=60324

If an `android.jar` file does *not* exist for a target platform we
should raise a normal error. Instead we throw a
`NullReferenceException`.

	…/xamarin-android/bin/Debug/lib/xamarin.android/xbuild/Xamarin/Android/Xamarin.Android.Common.targets: error : Error executing task GetJavaPlatformJar: Value cannot be null.
	Parameter name: path1
	Error executing task GetJavaPlatformJar: System.ArgumentNullException: Value cannot be null.
	Parameter name: path1
	  at System.IO.Path.Combine (System.String path1, System.String path2) [0x00003] in <4656b2b94f914437bce672312dd9e44b>:0 
	  at Xamarin.Android.Tasks.GetJavaPlatformJar.Execute () [0x00188] in <3fa51b5c8b554c02b4c7f549bcabde77>:0 
	  at Microsoft.Build.BuildEngine.TaskEngine.Execute () [0x00000] in <cbad9a49d2cc4e87ad2241615dd4666c>:0 
	  at Microsoft.Build.BuildEngine.BuildTask.Execute () [0x0008d] in <cbad9a49d2cc4e87ad2241615dd4666c>:0 

Oops.

The cause is that
`AndroidSdkInfo.TryGetPlatformDirectoryFromApiLevel()` can return
`null`, which is by design. We need to handle that in the places
where it's called, and provide an XA5207 error message when
`TryGetPlatformDirectoryFromApiLevel()` returns `null`.

src/Xamarin.Android.Build.Tasks/Tasks/GetJavaPlatformJar.cs

@@ -69,24 +69,16 @@ namespace Xamarin.Android.Tasks
 			}
 
 			platform            = GetTargetSdkVersion (platform, target_sdk);
-			JavaPlatformJarPath = Path.Combine (MonoAndroidHelper.AndroidSdk.TryGetPlatformDirectoryFromApiLevel (platform, MonoAndroidHelper.SupportedVersions), "android.jar");
-
-			if (!File.Exists (JavaPlatformJarPath)) {
-				Log.LogError ("Could not find android.jar for API Level {0}. " +
-						"This means the Android SDK platform for API Level {0} is not installed. " +
-						"Either install it in the Android SDK Manager (Tools > Open Android SDK Manager...), " +
-						"or change your Xamarin.Android project to target an API version that is installed. " +
-						"({1} missing.)",
-						platform, JavaPlatformJarPath);
-				return false;
-			}
+			JavaPlatformJarPath =  MonoAndroidHelper.TryGetAndroidJarPath (Log, platform);
+			if (JavaPlatformJarPath == null)
+				return !Log.HasLoggedErrors;
 
 			TargetSdkVersion = platform;
 
 			Log.LogDebugMessage ("  [Output] JavaPlatformJarPath: {0}", JavaPlatformJarPath);
 			Log.LogDebugMessage ("  [Output] TargetSdkVersion: {0}", TargetSdkVersion);
 
-			return true;
+			return !Log.HasLoggedErrors;
 		}
 
 		string GetTargetSdkVersion (string target, XAttribute target_sdk)

src/Xamarin.Android.Build.Tasks/Tasks/JarToXml.cs

@@ -68,12 +68,9 @@ namespace Xamarin.Android.Tasks
 			}
 
 			// Ensure that the user has the platform they are targeting installed
-			var jarpath = Path.Combine (MonoAndroidHelper.AndroidSdk.TryGetPlatformDirectoryFromApiLevel (AndroidApiLevel, MonoAndroidHelper.SupportedVersions), "android.jar");
-
-			if (!File.Exists (jarpath)) {
-				Log.LogError ("Could not find android.jar for API Level {0}.  This means the Android SDK platform for API Level {0} is not installed.  Either install it in the Android SDK Manager, or change your Android Bindings project to target an API version that is installed. ({1} missing.)", AndroidApiLevel, jarpath);
-				return false;
-			}
+			var jarpath = MonoAndroidHelper.TryGetAndroidJarPath (Log, AndroidApiLevel);
+			if (jarpath == null)
+				return !Log.HasLoggedErrors;
 
 			// Ensure that all requested jars exist
 			foreach (var jar in SourceJars)

src/Xamarin.Android.Build.Tasks/Utilities/MonoAndroidHelper.cs

@@ -530,5 +530,21 @@ namespace Xamarin.Android.Tasks
 			foreach (var ext in pathExts)
 				yield return Path.ChangeExtension (executable, ext);
 		}
+
+		public static string TryGetAndroidJarPath (TaskLoggingHelper log, string platform)
+		{
+			var platformPath = MonoAndroidHelper.AndroidSdk.TryGetPlatformDirectoryFromApiLevel (platform, MonoAndroidHelper.SupportedVersions);
+			if (platformPath == null) {
+				var expectedPath = MonoAndroidHelper.AndroidSdk.GetPlatformDirectoryFromId (platform);
+				log.LogError ("XA5207", "Could not find android.jar for API Level {0}. " +
+						"This means the Android SDK platform for API Level {0} is not installed. " +
+						"Either install it in the Android SDK Manager (Tools > Open Android SDK Manager...), " +
+						"or change your Xamarin.Android project to target an API version that is installed. " +
+						"({1} missing.)",
+						platform, Path.Combine (expectedPath, "android.jar"));
+				return null;
+			}
+			return Path.Combine (platformPath, "android.jar");
+		}
 	}
 }