Allow background registration and adds check for Activity nullability…

… in a more specific method.

packages/in_app_purchase/android/src/main/java/io/flutter/plugins/inapppurchase/InAppPurchasePlugin.java

@@ -37,8 +37,8 @@
 public class InAppPurchasePlugin implements MethodCallHandler {
   private static final String TAG = "InAppPurchasePlugin";
   private @Nullable BillingClient billingClient;
-  private final Activity activity;
-  private final Context context;
+  private final Registrar registrar;
+  private final Context applicationContext;
   private final MethodChannel channel;
 
   @VisibleForTesting
@@ -67,18 +67,14 @@ static final class MethodNames {
 
   /** Plugin registration. */
   public static void registerWith(Registrar registrar) {
-    final Activity activity = registrar.activity();
-    if (activity == null) {
-      return;
-    }
     final MethodChannel channel =
         new MethodChannel(registrar.messenger(), "plugins.flutter.io/in_app_purchase");
-    channel.setMethodCallHandler(new InAppPurchasePlugin(registrar.context(), activity, channel));
+    channel.setMethodCallHandler(new InAppPurchasePlugin(registrar, channel));
   }
 
-  public InAppPurchasePlugin(Context context, Activity activity, MethodChannel channel) {
-    this.context = context;
-    this.activity = activity;
+  public InAppPurchasePlugin(Registrar registrar, MethodChannel channel) {
+    this.applicationContext = registrar.context().getApplicationContext();
+    this.registrar = registrar;
     this.channel = channel;
   }
 
@@ -120,13 +116,13 @@ public void onMethodCall(MethodCall call, Result result) {
   /*package*/ InAppPurchasePlugin(@Nullable BillingClient billingClient, MethodChannel channel) {
     this.billingClient = billingClient;
     this.channel = channel;
-    this.context = null;
-    this.activity = null;
+    this.applicationContext = null;
+    this.registrar = null;
   }
 
   private void startConnection(final int handle, final Result result) {
     if (billingClient == null) {
-      billingClient = buildBillingClient(context, channel);
+      billingClient = buildBillingClient(applicationContext, channel);
     }
 
     billingClient.startConnection(
@@ -204,6 +200,15 @@ private void launchBillingFlow(String sku, @Nullable String accountId, Result re
           null);
       return;
     }
+    final Activity activity = registrar.activity();
+
+    if (activity == null) {
+      result.error(
+              "UNAVAILABLE",
+              "Details for sku " + sku + " are not available. This method requires to be run with the app in foreground.",
+              null);
+      return;
+    }
 
     BillingFlowParams.Builder paramsBuilder =
         BillingFlowParams.newBuilder().setSkuDetails(skuDetails);